Congratulations!
You've discovered the beauty of Practice Cards and Rule 5, "Keep your dear teacher happy!" First thing to do is to have three kids "pretend" to chew gum. You "pretend" like you are teaching, go get a practice card for Rule 5 and slip it into their card pocket chart. Then explain that if a child even looks like they are chewing gum, they get a Rule 5 card ... as the year goes on, of course, you increase the penalty from two minutes of practicing a rule at recess to three mintues and so forth. We created Rule 5 for exactly these kinds of situations ... you don't need a rule for every type of negative behavior. Rule 5 is one size fits everything.