should i send a client his final tax return before payment?

For review, send him a copy and electronic versions of w-2 and watermark the copy not for filing. If he is local, insist on a sit down review, or give him his documents back and say goodby.

New client? Apparently since you don't trust him or her!

If that is how little you trust your clients, perhaps you have the wrong client base.

This is a common problem for preparers that get random clients from who knows where, lots of people looking to scam you...you give them a copy and they never return and never pay you and use your numbers in some DIY program.

Many preparers collect a deposit up front prior to starting the preparation, just to make sure you dont end up working for free.

If therye going to make this an issue, and refuse to pay, Id return their workpapers back to them and they can go find someone else.  I wouldn't give them a copy until I was paid for my work.

"...A client insists..."

That alone would make this a 'former client'.   

Since I'm another Jeff, I will second the other Jeff's post.  You have to have a little trust in your clients.  If you don't trust them, do you really want to be doing their returns?  I have never held a client's return hostage for payment.  Maybe it's the anvils talking, but once I'm done with the return the client picks it up and pays me whenever he or she likes.  Some clients pay on the spot, some pay in a few days, and some pay in a few decades, but for the most part they pay.  Every once in a blue moon I write off an outstanding account but that is pretty rare.