If you're not serving data from a popular ad server like google/doubleclick there will always be a false positive or two, especially with things like hosts-based ad blockers that are extremely rudimentary but work ~60-70% of the time.

And if you manage to serve data from doubleclick then either you're working for them or something has gone horribly wrong. In either case just putting up a script to say "please use an ad blocker" is the least of your concerns.