Is it possible to limit download/upload, so only a few users can do it ?

Yes it is.
You - or whoever is the administrator of your AmpJuke installation - can edit the user(s) who should be able to download/upload music.
It's done by selecting "User adm..." (located in the lower left corner, provided you're an administrator).

When editing a user-profile, it is possible to check/uncheck if that individual user can/cannot download/upload.

12-07-2010