Is it legal to include a clause prohibiting a certain person from visiting?

It is perfectly legal. Reason being is that the landlord owns the property and should they have reason to believe or have proof that this person owes them, has stolen, damaged property, or that they just don't trust their actions -they can have them barred and tresping charges filed. Person A could go to jail for violating the terms of the agreement with the said lease holder. Person A in this doent appears to be a nomadic dweller. Will talk people into letting them stay, rent free while not on the lease. Person A staying there was actually committing a crime by not being on the lease, therefore landlord has the right to specify that Person A does not set foot on property. Check your Rental Laws for your state. I bet you will find the person above is right to an EXTENT, but that extenuating cirstances can be placed upon property should it come into dispute later on.