Removing the tag from the question should be sufficient. Tags are also automatically cleaned up if the tag exists on one question for some amount of time (I believe it is 6 months if I recall correctly). Banning and blacklisting the tag requires action from the Community Team. Given the way the tag is currently worded, I do not see a reason for having this tag. If there are questions about "sitting" that are related to some physical fitness routine, I would imagine there are likely better terms to use than just "sitting."
I considered your question off-topic because its wording sounded like it was purely about sitting. The connection to what your exercise routine was not there, but you've provided some context for it. I simply thought you were just asking about sitting in a chair in that particular position for a long amount of time. I don't think what you are asking is quite off-topic since you could make the stretch relevant to running or volleyball but not sitting. You are welcome to edit your question accordingly. However, in your other comment you do ask what the cause of your pain in your hip might be which is off-topic.