I am not an attorney but even if there is some weird clause in your lease I don't think it would be enforceable, especially in a city as tenant-friendly as Baltimore. That said, the landlord could be a petty asshole and try to hold your security deposit if he thought you were turning off any potential tenants so I guess ask yourself if you want the inconvenience. My guess is if they are making up this kind of stuff they'll probably find an excuse to try and keep your security deposit anyway.