You say, "I'm sorry this isnt working for me", delete and block all their contact details and curl up in a ball for a day, gorge on chocolate and then get up and move on.
If you're talking about a collared relationship, you give the collar back - unless they were kind enough to take it off you and release you from the obligation.
Was this your first D/s relationship? They can be hugely intense and depending on the level of control you may find yourself a bit lost for a few days but just remember... you cant give away what you don't own in the first place.