This is more specifically of individual one-on-one roleplays, but I really think people need to get more comfortable with saying, "I don't enjoy this anymore. I would like to quit." I think oftentimes people are worried about making the other person feel bad, but 8/10, they probably aren't enjoying either if things start to get dull. Even if they are enjoying it, I would hate to be in a one-sided roleplay, painfully dragging someone through a story they don't enjoy. It's just a matter of politely telling someone that you'd like to stop. Like, they can't hold you hostage. Just don't be rude and ghost people.

On the flip side, if you are enjoying a roleplay with someone but they aren't and ask to end it, don't beg them to stay or say you'll change the story/character/whatever. I've had this happen before, and was so annoying and embarrassing. 

In summary, just be kind, polite, and let people have their freedom