Assuming there is a clause in there in reguards to the lease being valid to any future owners of the property....
What was the deal between the property owner and the company that leased the wall?
If the deal was the property owner would get XXX% of income off the wall adspace, then the contract is still vaild if the wall has not had an advertiser.
If they are supposed to pay XXX.XX per month no matter what the display is, they are in breach of contract, and you have to notify them in writing that they have to bring the contract current or it will be null/void. Once you notify them in writing, with no reply, you should be good to go with whatever you want to do... You must send the letter certified mail and give them a time period to respond, check with your local laws for that time frame.