Any recurring pledge, payment, or subscription you make will have some terms and conditions on it about when and how you can cancel, and what kind of refund, if any, you can expect if you do. You would expect, though, that you cancancel — and that when you do, they properly stop billing you going forward.
Alas, both these things are not always true. You do, however, have rights. You should be able to make the payments stop — and if you can’t, you can get help.