Are you implying that the player deserves to be punished? Because it isn't terrible because your character thinks it is terrible, it is simply terrible for the player. Some of the /best/ rp to be had is when terrible things happen to your character, so there is definitely a difference here. Jail by itself is simply not fun, if ruining the fun is a illegal for villains...well you know. I just don't believe in holding non-villainous characters to a lower expectation of play. White Knights can be worse than villains some of the time, wouldn't you say?
If you really want to put in jail, put in some effort too. Put in the same effort as villains do, as people who play villains deserve as much effort as anyone else. Talk with the person, actively punish, or just plain interact. Isn't that the point, to interact? Besides, simply 'leaving' a character behind to play on an alt will completely miss the point of the punishment. I would argue that what makes roleplay worth while is the unique empathy that you have with your character. Empathy as a result of controlling your character, so if you want true character growth from a prison sentence, you have to be present, as a player. Try and make it fun.
However, I got a nice little idea if RP'ing with people you put in jail isn't your thing. Instead of jails, why not use stockades? They sure as hell are a harsh punishment for the character, but not so much for the player. I'd rather play a character stuck in one for three days, simply because I could interact, then spend one day in jail alone. Because, for me, if someone doesn't RP with me while my character is jail, I treat it like a ban, and simply won't play until the time is up.
Effectively starving a player of play as punishment because you don't like how RP went is a ridiculous notion, in my opinion.