New Jersey's Required E-File on CBT-100-S

Can I choose not to e-file a tax return for a client?
You cannot opt-out. Only taxpayers can choose to not have their tax return e-filed.

What if I have returns that cannot be e-filed?

The mandate doesn't apply to corporation returns, if that's what you're filing. 

What are the requirements of the E-File Mandate?
Paid tax preparers that prepare 11 or more New Jersey individual Income Tax resident returns including those filed for trusts and estates during the tax year must use electronic methods to file those returns.

If you're asking about an individual income tax return, there's this:

Most New Jersey Resident Tax Returns (NJ-1040 and NJ-1041) can be e-filed. If you cannot e-file a return, Form NJ-1040-O , the E-File Opt-Out Request Form, must be completed. You should enter a reason for your use of the form. Also, count these returns toward the threshold.

What if my client does not want to have his return electronically filed?
Clients who do not want their tax returns filed electronically can complete an E-File Opt-Out Request Form (Form NJ-1040-O ) so a paper return can be filed for them. You are required to send a copy of Form NJ-1040-O with the paper return, fill in the NJ-1040-O oval or box on the return, and keep a copy of the completed and signed form in your records.

https://www.state.nj.us/treasury/taxation/efilemandate-faqs.shtml