HopetonBrown's suggestion is what I use at the NRA Range. Other solutions I've used in the past:
- If I'm using a large range bag, I position it in front of the spot where magazines usually land and the bag stops them from going forward of the line.
- If the range provides cardboard backers for the target hangs, find a shot up backer and wedge it along the front of your booth so it stops anything from going forward of the line.
If you go to that range often enough and are really adventurous, you could cut a 2x4 about half an inch wider than the range booth's width and wedge it in there gently to catch mags from sliding forward. But seriously, HopetonBrown's solution is probably by far the easiest unless you're going to be doing a lot of reloads.