Unfortunatly border adhesive, once fully cured, is not water soluable. I've tried everything and have never found anything that will desolve it. I once read that club soda will soften it enough to be scraped off, but I've not had any success with that idea either. You may be forced to live with it or hang another border over it.
Sorry thats not better news. I'm hoping someone else here has better ideas.