Most sensible estate agents will have a list of people who ring up / enquire about properties. If this person is on that list (ie they enquired about your property) then the estate agent can / will say that they introduced that person to you. I would suggest (opinion only) that if you were to take it off for around 6 months then look at marketing it again, the estate agent wouldn't have a chance, but if you took it off and sold it privately before that the estate agent would probably send you a bill and sue you if you didn't pay.