Check to see if there is a recorded easement in favor of the developer/drain pipe user(s). If there is an easement, then they do not need your permission. If there is no easement then, tell them to remove the pipe.
Note: you have not stated when this pipe was placed. If the pipe has been there for many years and they can claim a prescriptive easement. If the pipe drains water onto your property, you have a different cause of action.