Using debris as a 'warp point denial system'
The problem is you can't do it at all, except on a closed warp point. The instant you dump the material into the space which would be occupied by an emerging starship, it goes through to the other side, where it simply becomes a debris cloud the ships pass through on their way into the warp point. The drive fields will give them plenty enough protection against that sort of normal-space impact on their way in.
Mines are not in the materialization area of the warp point; they are on the periphery, which is as close as they can get without literally making transit (whether they want to or not). To put the material into an area which would present a danger to emerging starships would require dumping it into the transit zone, which is one reason no starship has ever been lost to interpenetration with naturally occurring space junk making transit through an open warp point; the stuff simply can't stay even momentarily in a place where it would pose any danger to emerging ships.