update

See post below for initial details of weird asshole old guy.

After one of my students left, the guy said, "So are your office hours over yet?"

And I mumbled something like, "We'll see, I guess." Then, I said, louder, "Since you bring up my office hours, I'd appreciate it if you wouldn't interrupt me while I'm with a student." A request that I think is reasonable and appropriate.

Asshole says, "What, Vanderbilt doesn't give you people an office to do your shit in?"

Then, there was a stupid little "you were rude to me"; "no, you started talking to me first. you were rude to me." So, yeah, I could have handled it better.

To prevent a yelling match, I left to talk to the proprietor, Sam, about the escalating situation. This little asshole old guy followed me right up there, talking over me, and Sam said to him, "Just a minute."

Then, Sam talked to me about how I was a long-time, valued customer, and he'd take care of it. I went outside, Sam talked to the asshole, and the three of us sat together while Sam basically told us to play nice. Apparently, to the old guy, asking me to plug his computer in was less interruptive than just barging through. What would have been better would be for him to go around my chair and move the power strip so he could plug it in himself without bothering me.

While I appreciate the proprietor's need for diplomacy, the compromise he tried to create wasn't enough. I probably won't be coming here anymore. 

But at least the asshole old guy left.