I agree with Attorney Sterken, and I'll add, litigation and threats of litigation in cases like this needs to be addressed early and sternly. Take pics now, you'll likely need them later. You don't want them to keep improving the land and build evidence of an expanded easement while you're trying to figure out what to do.
You should contact a good real estate attorney in your local area immediately
It would depend on the nature and scope of the easement. Also, it would be helpful to know whether this type of access was occurring prior to the signage and improvements. It would appear based on the facts presented that the answer is not clear and would require legal action on your part to stop the activity if it is interfering with the quiet enjoyment of your property. The likelihood of success depends largely on the terms of the easement itself, and what has happened up to this point.