You're pretty much screwed either way because lying about it will always get people mad. Lying like that is going to get them labeled as a flake, tittybaby, liar or other labels that aren't very good either. Those labels will also earn someone namedropping and grudgewank. Honestly if someone is going to run to rpanons or wankgate about you and grudgewank about you, they don't need much of a reason. The best you can do is ensure that their namedrop will be scoffed at for being weak or lacking deets. If you actually lied to them, they will have deets to drop about being a liar and that is going to affect the perceptions of others more than being boring ever will. I know I'd probably give someone who is supposedly boring a shot, I'd not bother with a liar.

That's why "I'm just not feeling this thread" may be the best thing and just handwave a conclusion or see if you can skip to something better in the thread. If they are so bad you don't want to thread with them again then maybe a "I don't think our writing styles mesh well" is a nicer way to put it than "you're boring". If they are downright hideous and worthy of a namedrop (godmoding, infomodding, pushing stuff you didn't okay, etc.), screencap that shit and tell them never to tag you again.