If you come across that type of behavior I really do ask that you report them immediately under the Harassment report and describe what happened with the forced surrender. What they did is not allowed, nor does Riot tolerate such behavior.
There actually was a case exactly like this. Riot banned the players from that premade that would force surrender ranked games even when they were at the enemy nexus. Riot can detect this sort of harassment, so don't be afraid to report them.
I am really sorry you had to go through that. It's not fun nor is it fair to anyone :( Not all premades are rude, but that group was especially heinous, so I really do hope that you reported them so they can no longer force that upon other players D: