Prevent laptop bag strap from adjusting

Question

I have a laptop bag with a shoulder strap that I've adjusted to the right length, but over time it gets longer again due to bag weight. I'm looking for a hack to force it to stay in the same spot for my adjustment. The only thing I've thought of myself is to use a safety pin to pin both parts of the strap that overlap near the end to force them together in the same spot.

The kicker here is that my bag's strap cannot be removed from the bag. It does not clip onto D-rings, it's sewed around the D-rings on the end and sewed into a loop, so I cannot actually remove the strap to wrap anything around it (e.g. a rubber band).

Here's a picture of a similar bag with a similar strap configuration (again, not the exact bag I have but here for reference).

Answer 1

Permanent fix, (although if you do it right you can undo and redo if adjustment is needed):
Sew through the part of the strap between where you adjust it and the D-ring below.
If you do only a couple of stitches, it will be easy to undo but does run a risk of coming out when not wanted.
For a permanent fix which is almost fail proof you will need to do a lot of stitching and maybe even have it done on a professional sewing machine, like by a shoe repair shop.

A more temporary and more easy to adjust fix can be done with a safety pin, as you had thought out yourself. But do get the kind that are made for nappies/diapers, which will not go open without extra action. (I have had pin pricks in my wrists from my back-pack adjustment stopping pin.)
An other alternative (if you are the right kind of person for it) is a pin-back button which is more something for a student than for a businessman.

Answer 2

A quick squeeze of hot melt will keep it from slipping. Also easy to remove in the future should you want to make other adjustments.