Can the buyer agreement be changed after once agreed

We are a Real Estate Company, engaged in Construction Of Building. We sell the buildings to prospective customers following all laws on nation. Our relation to customers are well documented. We have observed that some of the customers calls repeatedly even after office hours and enter to long conversation. Most of them are beyond agreemental provisions. Most of those are unnecessary/ items to be complied at their end. These affects our works as well as privacy. Is it possible to include clauses under the Agreement, so that it should be possible for us to initiate cases against them if they repeatedly disturb us. If yes, please let us know Charges as well as time frame.