Suilad Elrowen,
the best place to learn Sindarin is
www.councilofelrond.com they have a very good sindarin course and the mods will mark your work/give pointers and there are those willing to mentor people...
the closest I can get for you is be silent which is:
Avo pedo or No dínen (both lit: Silence!)
Or for a little extra how about
:
Mîbo orch! (go kiss an orc!)