It is simply do missions with other bad sports.
I have seen this happen twice now. Once with my friend the other day. He had 1 day left and we were doing some rooftop together then after about 4-5 I wasn't able to invite him to the game any more. He messaged me saying I was a bad sport and his dunce cap was gone.
Same thing happened to me today, I had a day left as well. Same scenario doing rooftop and I kept inviting the same random guy to my game. After about 4-5 sessions of rooftop I tried to invite him again and it said he was a bad sport. I checked my character and the dunce was gone.
I thought about it and it makes sense to have a way to get out of bad sport faster and what better way then to use team work and be a productive member of society. It's like jail, if you be good and work on becoming a better person your time can be adjusted.
If anyone is badsport try this an chime in if you notice time being reduced.