Oh, please do track him down and give it to him! And get someone to YouTube his reaction. 
Now that looks like it could hold letters.
Being a physics teacher, I deal with g*ddamn slinkies every year when we study waves. As I was reading this thread (not realizing it was old), I was thinking “I know the perfect thing to do!” So I’ll post it in case it comes in handy to anyone who finds themselves in a similar situation as the OP.
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Overstretch the slinky. Get an accomplice and each of you take an end (hold it tight!) and walk away from each other, stretching it out past its normal tolerance. When it’s been properly overstretched, you and your accomplice can walk back toward each other and the slinky’s coils won’t naturally pull it together. It’ll probably be ~20 feet long or more now when relaxed.
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Compress the slinky. This doesn’t require much force, but it can be tricky because the coils will try to bulge out laterally. Maybe start with one end and just gather the coils a few at a time toward you until you’ve got the whole thing compressed so that it looks like a normal, unstretched slinky.
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Hi, Opal!
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Secure the slinky. Use some strong rubber bands or some wire to wrap around the ends to hold the slinky together. Make sure it can’t bulge out sideways.
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Place the slinky on colleague’s desk and wait for him to unwrap it. Surprise!
(it’s like the old snake-in-a-can prank)
What should I do with an annoying slinky?
What should I do with an annoying slinky?
What should I do with an annoying slinky?
Ear-lie in the morning?
Kink it every morning when he isn’t looking,
Kink it every morning when he isn’t looking,
Kink it every morning when he isn’t looking,
Ear-lie in the morning!
Chorus:
What should I do with an annoying slinky?
What should I do with an annoying slinky?
What should I do with an annoying slinky?
Ear-lie in the morning?
Bury it in Jello when he’s gone to dinner,
etc.
Ear-lie in the morning!
Chorus
Hide it in the ceiling where he’ll never find it,
etc.
Ear-lie in the morning!
Chorus
Spear it on a cactus so he will not touch it,
etc.
Ear-lie in the morning!
Chorus
Make a polyurethane base for it,
etc.
Ear-lie in the morning!
Chorus
Rust it shut with water and a wee small voltage,
etc.
Ear-lie in the morning!
Chorus
Stretch it and bind it and let it scare him,
etc.
Ear-lie in the morning!
Chorus
That’s what we’ll do with the annoying slinky,
etc.
Ear-lie in the morning!
I feel like you, someone that sits near me clicks his pen…constantly. I can hear him when he’s approaching the desk, I can hear him when I’m out in the store and he’s walking past me. About once a month I’ll half jokingly/half sternly say ‘knock it off with the fucking pen’ or if I’m on the phone and getting frustrated I’ll just grab it out of his hand and toss it across the room. It only works for a day or so, but he knows it bugs everyone. That would have been my suggestion with the slinky. Just say ‘knock it off’ and see what he does. If you say it in such a way that he can’t tell if you’re mad or joking and enough other people hear you, he might stop.
I realize five years have passed, but I just had to search for it. So, here it is: Escalator Slinky.
Where were you 5 years ago!!! That would have been the PERFECT solution! Especially since I had zip tied it shut a few times prior. I wouldn’t have been surprised to see it secured…
Oh, I would have loved to see him get punched in the face by his own slinky!
Thanks for the compliments. I’m pretty pleased how it came out. I just don’t know if I can part with it now. And, while I do know where he works - I don’t know anyone else there that could record his reaction.
I am considering sending it to him. Just haven’t decided.
I guess its way too late, but with the angle cutter, you could have clipped off 1-2 loops of metal a day before he got to work or after he left for the day. The slinky would have been gone in no time. 
“Hey Bob, are you feeding it? It looks like its losing weight…”
Not to mention that if you’re crafty, you could have made hoop earings out of the parts for the office X-mas party…
Hum the slinky song all day long.
I’m curious as to how he reacted when you took it? Did he raise a fuss in the office about who stole it? Did he accuse anyone? Did he say anything at all? How long was it before he bought a new Slinky, or did he just do without? Did other people say anything? How much longer did you stay working there? Did the Missing Slinky become an office joke?
Ha. I like the idea of the shrinking slinky. I wonder how long it would have taken for him to notice?
Kittenblue - He said nothing. He completely didn’t react to it. I really did plan on returning it in some pranked form, but he never brought it up. I was happy to just do nothing until he complained. He never did.
It was probably 2 months later that the company cut 25% of the work force. This was right in the middle of the recession and both he and I got laid off. I haven’t talked to him since.
Hide an Eviltron somewhere in his cubby. Guaranteed to drive him insane in short notice (it almost did my nephew when I sprung it on him).