The only polite way is to lay on the horn non-stop while following them to their destination (so they know you're there), when they stop, get out, approach them as quickly as possible (to be respectful of their time) to thank them for going
Yup. A quick tap of the horn or double tap is fine. Even a 1 second blast. Once we get into 2 seconds blast territory, it transitions to rude, with 3+ seconds going up linearly on the deranged jackass scale
Sadly horn volume is regulated with the intention of preventing things like loud air horns from being used on personal vehicles.
I saw some video of an infuriatingly positive and giggly guy mod his horn this way. To add insult to injury iirc (which is about 50/50 after so many years) he used an arduino to do it. Video got a gazillion likes and many supporting comments. It unexpectadly became the last straw before deleting my facebook account.
I know… I’m no fun.
A solid 10 second blast and a flash of your gun to let em know no funny business.
Ya know, sane stuff