RTFM, Be nice or not

I have a product that serves end customers and brings me some money. The software is mature, being used by tens of thousands of customers and I had zero real bugs reported during the last couple of years. Still I get support emails that are way too often undecipherable, offensive, patronizing etc.

I am trying to be cool and helpful and always answer those even though the answers are just a reference from the user guide, FAQ or generic help as people can't use their computers.

Curious at what point I should just tell them to read the effing manual/FAQ. Normally I would dedicate support person but the revenue is not enough for that. Shutting down is not an option either as it brings reasonable and steady profit.