In PostScript Level 2, you can use the undefinefont operator to remove fonts selectively.
The only reliable method in PostScript Level 1 is to restart the printer, for example with:
serverdict begin 0 exitserver systemdict /quit get exec
Of course, the real solution is just to not download persistent fonts unless you really want them to persist indefinitely. If you want them only for one job, download them as part of the job.