When you go to the Civic center you can complain about a resident, I've never tried it and I'm really curious as to what happens, but I don't actually want to do it.

The main complaints I know of are about their language and their clothing - this is so you can remove any inappropriate designs or catchphrases. I'm not sure it actually affects your friendship with the villager.

"Rude: This changes their 'catchphrase', a great way to fix a character if someone came in and programmed them to say something you really don't like. It simply resets it.

Slobby: This changes the clothes the villager is wearing. Another friend of mine had someone stick an offensive pattern in her Able's, and Peanut was strutting around with it on. After selecting 'Slobby', Peanut changed her outfit and she purged Able's of the pattern.

Weird letters: For when someone has sent a neighbor an offensive letter that the villager is now showing off to people. It purges their memory of all letters received."

From a website I found