What to say to a bad road user

If they are behind you sounding their horn then why do you have to say anything? As Hanson says, you need to concentrate on what you are doing. The chances of being able to have a rational conversation with them are small. As long as you know you are cycling in a safe and considerate way then why let their rudeness and impatience bother you? Only if you think they are going to ram you should you pull over and let them through.

Exactly.

Additional food for thought:
Despite the huge differences in effective levels of danger, damaging, attempting to damage, or even abusing in any way a driver's vehicle is likely to provoke a similar reaction to if a driver / ped were to walk up and key your lovely carbon/ti/steel/magnesium/bamboo/gas pipe weekend warrior build, and shouldn't be undertaken unless you are in danger, and even then should not be carried out with the intention of causing damage, rather to alert said driver or deter them. It may sound obvious, but does the 'them or us' attitude often propagated on angry threads about crazy drivers actually make this come across as a good idea? Is d-locking the wing mirror of a driver who passed a bit close than you would like the answer? Logic says no.