I don’t mind sharing a chapstick with family members or close friends, or a water bottle, either, but I draw the line at a chapstick used by unknown persons. So far, I’ve never had a fever blister and I’d like to keep it that way. I’m not sure a chapstick can transmit the virus but it’s not worth the risk to me. I tend to keep my chapstick in my pocket, anyway. Luckily, they are pretty cheap to replace.
Before you leave for the day, dip your chapstick in one of the following:
- Oleoresin capsicum or pure capsaicin
- Powdered alum
- Ipecac
Make sure you buy a new tube before returning to work.
I would not want anyone using my chapstick that I am not willing to kiss on the mouth.
I’ll make exceptions for SOs and my parents (no, I do not make out with my parents), and probably my best friends but I wouldn’t let my brother use my chapstick, let alone a cube mate! Eew!
But then you’d have Chapstick all over your butt. I think that would be a big enough drawback to not go down the particular revenge avenue.
Maybe the user thinks it’s some sort of company-provided Chapstick, and that everybody has a tube at their desk.
Or, for those who are where they can’t watch a video: Ass Pennies
What emergency requires brushing one’s teeth immediately?
Wait. Someone else sits at your desk after you go home? Ewwwwwwwwww.
Joke. The Chapstick thing is disgusting.
Ever eaten some really bad…risotto? 
You could always engage in schadenfreude. By this I mean, write them a note, asking why they are using your chapstick? Mention something along the lines of “I hope you haven’t been using this on your mouth!” and add “If so, I need to know if you get cold sores, because the last thing I want is cold sores there.” Let them wonder where there is, and get more grossed out than you are. 
I guarentee, it will cure the borrower of ever using a chapstick that isn’t theirs again.
You could apply some kind of superglue to the Chap Stick before you leave for the day, then when your culprit comes in to use it, they’ll have the Chap Stick stuck to their hand for a really long time.
I didn’t make the original Ass Pennies reference, so no. Besides, I think there would be meltage issues.
I suppose you could use it on the O-ring to keep from cracking in the winter. I guess it depends on how evil you wish to be.
My guess is that really would chap his ass.
Crisco would look more realistic. Bitter apple flavored Crisco. 
You could leave a note similar to what’s already been mentioned only put it this way:
I’m sorry you used my my Chapstick. I’m dealing with a huge cold sore right now…
::golf clap::
Maybe he saw you in your fairy uniform. Fairy chapstick is magic.
Bah. I knew I should have got there quicker. 