How do I label fluorescent fixtures converted to LED?
Don't say what shouldn't be in there, say what should.  "Direct-wire LED tubes only" 

I put the sticker directly behind (above) the tube so it hides the sticker.  I would place it near the lampholder which has the 120V on it, if that's a factor. 

Then on the endcap of the fixture, at the power cord end, "Ballast removed. Direct wired for LED."

If the fixture doesn't have a power plug (or if it's annoyingly inaccessible), my practice is to install an internal disconnect inside the fixture (scroll down and look for the orange connector) which can be had cheaply here.  It's not required for direct-wire LEDs (and several other exceptions) but it's a good thing to do, and the connectors are only a buck.