A) Have them do what any normal person would do and continue living their lives while supporting their friends and helping them through their own problems.
B) Get rid of them.
If it were just me asking this of myself, I'd do what @joannekwan suggested and consider giving them a new arc, but that's because I actually trust myself to know what I'm doing. ^^; I don't write new arcs that contradict earlier arcs, or those that serve to pointlessly extend a story that should probably just end. Or those that are just so trivial they serve to make their star characters feel annoying, not interesting.
Those three examples are things that I see other writers do a lot...which is why I would be very careful with suggesting to someone else that they just "write a new arc". :T Honestly, if you're inexperienced enough to feel genuinely clueless about what to do when a character finishes an arc, you're much more likely to fall into one of those traps if you try to give them another.
Suggestions A and B, however, will at least prevent you from ruining the character: either learn how to make a character engaging WITHOUT having them go through turmoil (something really important that you probably need to learn anyway) or have them gracefully exit the story somehow, so you can focus on the characters you do know how to handle.
And in the meantime, keep working on the overall story. I mean, if you don't like being in situations where characters finish their arcs early, you can...write stories where the character arcs actually span the whole story. ^^; Or at least character arcs that are synchronized, so all the characters finish their arcs at the same time (and then maybe they'll all be ready to face the big finale together).
Don't feel like you have to struggle through writing stories with events you don't actually want in them-- you're the author; your stories should be tailored to fit your desires. So, even if it's behind the scenes, you gotta work on them until they are.