There have been some long running threads on this subject in the past. Most agree that a rematch is neither compulsory nor a sign of good manners but a vocal minority differ. I'd just block them and move on with my life...
Rematches and Etiquette
People never complain about this with me, I almost always play only 1 match. When the opponent offers a rematch, I often accept, but that's just once in a blue moon.
My guess is that some people desperately want to play you more, since you're a titled player. More fun to beat a titled player, maybe sniff out your own chances for a title.
Also, I'm guessing you draw some spectators at your level, which may add a bit of a 'spotlight' to a match and add attention to who is playing who.

FIDE rulez of chess:
21.5.4. A player is obliged to accept unlimited number of rematches unless he's a total coward and worthless piece of garbage.
21.5.5. A player whose rematch request has been declined is allowed to call his opponent a coward (twice), a turnip (once), a donkey (once), Gandalf's stinky armpit (once, unless the game ended by stalemate (see 21.4.8) )
21.5.6. A player whose rematch request has been declined shall be compenstated by FIDE with new diapers and pacifiers.

I think it's good manners to accept a rematch if you won the first game, with the white pieces. Plus, what's the downside of accepting a rematch?
Have any of you guys noticed that people get insanely mad if you play one game against them and leave without playing a rematch? I see all these nasty notes left both on my profile and other people's profiles because of this. I don't understand why people think they are entitled to rematches or how you are a coward if you just don't want to play another game against them. Anyone else share this sentiment?