It’s a little different then what you describe here. It’s usually one-on-one conversations. And it’s not so much that he’s a talker, and can’t shutup. I think he just misunderstands my conversation enders and body language (that basically says, “Ok dude, stop looking at me, we’re done talking”).
I say things very simliar to what Sam Stone had mentioned:
I’ve been saying these things for years, people usually get it.
I feel all self-conscious now because I know I do this sometimes. I don’t want to do it, but I there are times when I miss the cues. It’s not like there is a standard and unambiguous way to signal the end of the conversation. (I know there are common ways to do it, but it seems like different people attach different weights to different signals.)
Well, actually, I suppose there are unambiguous ways to end a conversation, but they tend to be blunt and come off as rude.
Are you my sister-in-law? my MIL is like this, but without the staring bit. The cruelest thing in the world would be to put her in a room all by herself. I have to deal with a guy tangentially related to my lab. He will go on and on, repeating the same things three or four times. I had to talk with him recently, and I managed to keep the call to about 15 minutes of useful dialogue. Otherwise it would have been about 40 minutes.
I used to go on, but I learned to finish up a conversation. For me, the close is something like summing up what was said, relating the current exchange to the original topic and summarizing, making a verbal to do list, or saying “I’ll go get that started right now.”
All I can suggest are more obvious clues. I’m not always great about picking up on subtle hints about things. I don’t see anything wrong with “I need to end our conversation now because I’m working on this spreadsheet.” If it’s a work related conversation, maybe ask him “Do we need to schedule time on the calendar to talk about this tomorrow?”