If it was harassment or troll I would definitely ignore it (or maybe just have a bit of fun with them too ;-) OK that wouldn't be professional, but it does relieve stress.
But it's usually people that expect something for nothing, or just do not listen to the responses you give. After a bit of explaining the same thing multiple times, and getting the same answer back without it going any further, is where one starts to get frustrated, and a quick jab can sometimes wake them up.