You can't kick the players that already have it, thats nearly impossible. For me to have an event with 20-30 players in it, I would need to:
1. Find everyone who already has the weapon (how would I do that? Ask and people would lie?)
2. At the start, type /kick <acc name> about 20 times, which would take about 1-2 mins and I wouldn't even be able to watch to make sure people are following rules.
But I do agree Typhoon, it is a bad rule.