I want to convert my fiberglass tub enclosure to a shower?

First of all in your remodel, you'd should have to remove the TUB plumbing fixtures back into the wall cavity and cap all of them, but the inlet knobs, cold/hot and the shower head. Your SINGLE might have to go back to a double? I'd replace it anyway in a remodel of that scope. Then proceed with Concrete backer board and tile work, perhaps adding vapor barrier.

I think you should expect to change all the fixturing as well.

Just my 2 "sense"

OK you are going to tile your walls. With cement board and ceramic tile I assume. So this means you will be exposing the plumbing pipes.

The simplest and easiest way is to cap off the faucet with either a cap or a plug. Then the water only has one place to go -out the shower. Make sure you test it before you close up the walls again.

I agree with DIY Doc, as long as you have it all open why not replace the faucet etc. They are relatively cheap compared to the entire process.

A half inch line comes out of the bottom for the spout. You can solder on a 1/2 " cap on that or if it unscrews, use a 1/2" plug in the bottom of the faucet. Make sure you use Teflon tape on any threads.