If you have a concrete curb by your mailbox, it might work to do what I once did to keep kids from doing burnouts in my grass along the edge of the road; use a hammer-drill to drill 5/8" holes about 4" deep in the concrete every 8'. Just a few inches from the inside edge of the curb. Cut 10" long pieces of 1/2" rebar and drop in each hole. Cut 8" long pieces of "decorative" 4x4s (bevel off the tops at a pretty 45deg angle), drill 5/8" holes up from the bottom at least 6" and drop the 4x4s over each piece of rebar.
The little decorative posts alone will discourage running up in the grass or even cutting up close by the mailbox, since the fun-seeking hooligan driver canot be sure just how solid they may be. If one DOES decide that they would be fun to run over, the 4x4 will easily shred off leaving the rebar to shred tires.
IMHO, you'd be somewhat insulated against liability concerns with this design because 1) they are just "decorative" - the rebar is just there to hold them in place, and not for any sinister destructive purpose, and 2) there's little likelihood of any serious bodily harm. even in the worst case.