You can remove it. The easement is for passage, not advertising.

Here's a potential non-combative way out for you (in case your rear neighbor is a dick): A sign posted in an easement could be seen as an obstruction to clear and safe passage, and that's certainly not something you could permit, because it could impact all of the other users, and leave you liable. It is the responsibility of the owner (subservient property) to insure that the easement is not obstructed or restricted in a way that hinders the dominant users (them). If the sign is so far off the road that it couldn't possibly hinder two cars from passing each other, then it is probably no longer even in the easement.