Use a small zip tie under the gas button.

I took a very small piece of duct tape and rolled it into a small tube shape, bent that into a U shape, trimmed, and wrapped it around under the gas button of a mini-Bic. Held it in place with another small piece of duct tape circling the head of the lighter. The zip-tie others have suggested sounds better, except that would leave a bump at the locking part of the zip-tie which seems like it would snag on things. Then I used ranger bands (cut up bicycle inner tube) to attach the lighter to a P-51 can opener. The hole in the P-51 attaches to my keyring with a small split ring. The lighter pins the hook part of the P-51 safely between the lighter body and the back of the P-51. The head of the lighter and the hook end of the P-51 are on the end away from the keyring.

The cases I have seen for lighters that attach to your keyring all doubled or tripled the size of the lighter, which was not what I wanted. So far my homemade setup has worked perfectly, but I'll admit it looks a bit funky!

Ingenious solution! Can we see any images?

I "borrowed" one of my daughter's black elastic hair ties and wrapped it around my lighter under the gas button.

After wrapping it several times the tie builds up enough to keep the button from depressing.

It's easy to remove and easy to reinstall.

That is pretty much what I do for my every-day carry mini-Bic. So far, after several years riding in my pocket with very occasional use, the lighter is still mostly full. Seems to be working.

i just wrap a rubberband under the thingie...

Sorry, I don't have any way to display an image. It's nothing impressive anyway. Just a Bic lighter held onto a military-style P-51 can opener with some black runner bands.