I would your local councillor. There is a type of clear varnish that will protect paint. I heard the council may think it hazardous, but it is widely available and will have been put through all the right tests, so I don't buy it. (If you search it online you will find it is about ?25. I think a few cans of this paid for by Cleaner, Greener Southwark would be useful.) Glad that you informed the police because they may be able to build up a profile. FYI, the council will remove tagging from property that is private if you sign a waiver: 020 7525 2000 for the environment department, I think.