I assume there is a way to modify the Network Extender through your online account.
The user guide has this posted under the features area:
To prevent unauthorized users from accessing the Network Extender and diminishing your available bandwidth, you have the option to restrict the use of your Network Extender by logging into My Verizon at www.verizonwireless.com.
Try signing into your account and check out the area that lets you manage your devices. I'd assume if there will be some sort of area that would allow you to specifically block/black list specific numbers or allow/white list others. Wish I had the entire solution for you but this should be a good place to start looking.