You can delete a session by going to Qbank > Previous Sessions, selecting the 3 dots next to a session, and then "Delete":
If you delete some question sessions, the results of the overall statistics will stay the same. Only the session itself (visible in the overview) will be deleted permanently. Your overall statistics include every question, regardless of how many times you have answered it.
Your Study recommendations, as well as your evaluation, will only include every question once.