No it is not against the rules.
If they refuse of course is their right and maybe very frustrating as most good players can see a lost position and do the sportsman thing and resign.
That said I have seen and played a few where the opponent tries for the 50 move rule which in online chess can be forever but it is a legal chess strategy. Cheers & Good Luck
Is it against chess.com rules to politely ask your opponent to please resign a game when they are clearly going to lose? I realize you can't force them, but can you nicely ask them?
If not, I would love to see a link to the rules stating this.