A Qtip with some blue or red loctite, from the inside of the nosecone. Be sure to leave it open, with no refill for a day or so, so you dont glue the refill in. This happens with the sierra in a very tiny percentage of pens, but it is annoying when you get the ones that work free.
I’ve found that lots of people are surprisingly cleaver at ruining Sierras. I now Lok Tite the transmission together but not the whole transmission to the nib assembly. For that I use a small slipjaw pliers to tighten it firmly. That solves most of the fidget folks, but some will try to twist the front causing the issue you had.
I watched in mild horror as I talked with one customer as another customer wrenched one of my expensive Triton convertibles trying to open it. I paused my conversation and showed them how to unscrew a fountain pen; they tried five gyrations hard before I got to them. Expect weird stuff.