In fact, I have a little trick I often use.
We have 120 stations in our office and we met this issue so often. Our employees have 5 to 10 network mapped drives. What I did do fix the problem of mounted drives letters issues is that I set the CD-ROM drive letter to Z: so that I freed one drive letter (in our case E
and so they could use their USB drives and keys and it always assign that drive letter. We informed everyone not to use that drive and that's it!