I have drafted 3 strangers in 2 years of riding so not a common occurrence, normally there is enough of a difference (either they are faster or I am) that it isn’t a problem. Just occasionally you come across someone who is about the same on the same bit of road as you so you can’t just fly past them, or you have to stop for a bit to let them get away.
As for announcing I have said hi but it is pretty hard to be heard at speed, either that or I’m just been ignored.
I’m far more worried by being hit by a car than some chap drafting me. After all as the rider in front is likely to be your own fault by braking hard at a random point. How many accidents have you had from a rider drafting you?