I usually end up saying something like HEY! But it comes out Heeeeyyyy with a sharp Tuh sound on the end...Like Hate. Or AAAAAAT really gruff and deep like I swallowed a drink of gravel.
If need be I'll use the end of a lead to disengage their hiney or if they are at liberty...I try to puff up myself and stand real tall and say no.
I would never kick my horse...I would feel really silly getting into a kicking contest and losing!!!
My horses usually don't give me all that much grief. Guess I'm lucky.