We spray the cistern tops with WD40. It has an added advantage that if it doesn't deter the scrotes from sniffing the coke in the first place it induces nasal haemorrhaging. 
A sticker above any likely surfaces saying you do that, but not actually doing it, has been shown to be much more effective.
I think the best one was a "No snorting" sign (with a picture similar to the no smoking sign) and the text "All surfaces treated with drug neutralising chemicals, feel free to waste your drugs."
