You need to put a big sign next to the stakes for the first target that reads
ONE ARROW PER TARGET!
It cost a club about $3 in target replacement cost alone to send a shooter through a 30 target course. last month, we had a group go through and empty their quivers into every target out. Not only did they slow play (we had 97 shooters) but calculations indicate they cost the club $15 to $18 each, and paid $10 for Registration.
Guys will chew up you course with irresponsible behaviors if you let them, then, they will be the first to complain about poor condition targets.
If they ignore the sign and do what those Bozos did, kick them off the course immediately, and if they come back and do it again, Ban them permanently. You are better off without them, and spread the word about what happened, to discourage that kind of irresponsible behavior.