Found an answer that works for me:
warning: this will find and kill long running processes
ps -eo uid,pid,etime | egrep '^ *user-id' | egrep ' ([0-9]+-)?([0-9]{2}:?){3}' | awk '{print $2}' | xargs -I{} kill {}
(Where user-id is a specific user’s ID with long-running processes.)
The second regular expression matches the a time that has an optional days figure, followed by an hour, minute, and second component, and so is at least one hour in length.