All of us have our pet peeves that find their way into our classroom rules and procedures, and it varies from person to person. For example, I once worked with a math teacher who had an explicit "No Singing" rule. Apparently year after year she's had trouble with students spontaneously breaking out into song in the middle of class, and she reached her limit. I'm not sure if I haven't experienced it at the level that she has, or that it doesn't bother me, or that I just don't notice it, but I've never felt the impulse to impose an absolute ban (there have of course been isolated incidents wherein I discouraged it, like the Fergie/Jesus episode or in the wake of the release of Frozen).
However, there are 2 1/2 physical items that I have imposed an absolute ban on.
It is the herpes of decoration. It never goes away. You think it's all cleared up and then BAM another outbreak.
2: CANDY CANES
I HATE them. They are delicious and festive, but they always always break when students have them and they always always shatter into a bazillion sticky pieces and the students always always step on or smash them even further and they never never clean them up. They're like glitter that attracts bugs and vermin. Fun story: sometime in mid-April one year a senior walked into my room to deliver something from the office while eating a candy cane. I hollered: "Freeze! Back up to the door! You cannot have that in here!" He looked at me and the rest of the class in complete bewilderment. Several students backed me up and told him "Yeah, she's not kidding, you can't be in here with that." I love it when students vehemently defend your arbitrary rules.
2.5: MICROWAVE POPCORN
Only allowed after school, never before or during (students aren't the problem with this one, it's teachers. They really don't like being turned away. There is a teacher's lounge, people!). The aroma of popcorn smells heavenly the first ten minutes. As it continues to hang in there air, however, it quickly degrades into a weird funk that inspires every. single. student. that walks through the door for the rest of the day to loudly announce "It smells nasty in here!"
Those are my absolute bans. What are yours?