Priority can be set for the first 50 users you set. Those get to use the available 6 channels first. The 7th is for emergency calls only. You cannot block any numbers from using the extender. I have seen several complaints about other users, especially in crowded areas like apartment complexes who kept getting connected even without the #48 code. My phone would display the extender symbol (little house) even when I wasn't connected and was confusing to other people around me. Verizon was able to turn down the power of mine to avoid too much interference, but it was unavoidable.
Configure Extender in My Verizon