Unwelcome visits from Real Estate Brokers (to your tenants) no longer representing your building.

One of the many businesses I am involved in is a small (25K sq ft) building which currently has about 15 tenants. We have always had a real estate broker represent us and the building. A little over a year ago our old  broker let us know they thought we were too small an entity and were only willing to continue to represent us if business came to them. They wished to farm instead of hunt for us. We declined the opportunity to lose potential leads and formally switched to a new real estate broker. That broker is prominently displayed via a large sign at the entrance to our building.

Recently a sales person from the old broker came to one of our tenants they had worked with originally to see about working with them on their lease renewal. So far, no issue. The tenant has the right to hire whoever they like as we have the right to work with who we choose. The problem is that the tenant was turned off by this and does not want to be contacted by them - although he told us the broker assured him it would not cost him anything. They have continued to contact him even though he has told them he would re-negotiate on his own and not with them.

Is this practice of contuing to push even after a prospect has told you "No" a problem in the industry? Is there something I as a landlord can do to stop this? Would I end up legally responsible for any commission if a leasing agent tried to back door with any of my other clients? We don't want to upset our tenants, but we are disgusted with the behaviour of this sales person and we would like it to stop.