Full scale pilot gone berzerk
I think our club was worried about the liability issue if this guy (or someone else using our model runway) got hurt, or worse, got killed. You know how these jokers love to do things they shouldn't, wind up hurting themselves and then they (or their relatives) sue YOU like it was YOUR fault! Doesn't matter if you wind up being found not at fault - it still costs to defend yourself, (or the club in this case). The point about the "X's" was, according to the FAA, this put the fault on the person(s) ignoring the indication that this was a closed runway.