Snipers, point an obvious howitzer down the bridge.
Seeing a huge tube pointed at them might discourage shenanigans.
IIRC, Obama opened the doors to caravans a couple years back....thousands crossed without restraint.
Canister rounds.
Just a very few 105s with beehive rounds. They did a great job back in ‘68. Maybe there is something better today but nothing more intimidating than the aftermath of one of those rounds. Horrible.