"Stop" Means "STOP"

Recently, I was teaching in a 5th/6th grade combination class, where a group of girls were acting silly (not unusual)  and directing their silliness at various other girls.  Some played along and had fun, but it clearly annoyed one girl.  She first politely asked them to stop, but they didn't.  She attempted to physically remove herself, (we were lining up to go somewhere), but that also didn't work.  Finally, albeit a bit late in the game, I realized that this was a good opportunity to give the kids the message that "Stop" means "STOP".  So, I told the girls who were being silly that when someone says the word "Stop", it means that they need to "STOP".  They weren't actually bullying anyone with their silliness (I think it was something like just saying a goofy word over and over again).  I told them that, as long as the people around them didn't use the word "Stop", their silliness was just annoying, but once someone asked them to "Stop", then they needed to do so around that person.  "Stop" means "STOP".

The young girl who was the target of this, but didn't like it, looked at me with extreme gratitude and it occurred to me that I should have been doing this MUCH more often.  I can't believe it has taken me so long to get here.  I guess it is because, in my family we would tease each other quite a bit.  Sometimes "Stop" really meant - "This is funny and if it doesn't get worse, I am actually enjoying the attention."  The problem is, the person doing the action isn't always aware of the point at which "Stop" ceases to be funny and crosses over to "STOP" meaning "This is enough."

Now I need to think of a word to use that means "This is funny and fun, but be careful, because you are getting close to my limits."

"No" means "NO" and "Stop" means "STOP". What would be a good word for "Watch out; I am enjoying this now, but I may have enough of it in a minute."  -- "Careful"?  "OK"?