Tenant painting without permission

Other

Hello,
I have walked past my property a few times recently and keep noticing the tenant is painting the outside of the house. Some of the paint is on untreated brickwork which clearly will take a lot of labour to restore back to how it was. In the lease it states that no painting or decorating is to be done whatsoever (I used the stock RLA lease), but I did mention verbally that they could decorate the interior with my written permission (which they have not asked for and do not have). I am worried that if they paint the whole house it might push the restoration cost over the deposit sum. Is there anything I can do to stop them decorating? I have tried asking politely twice to leave the outside alone, but they have ignored me on both occasions.
Any guidance would be much appreciated,
Thanks in advance.