Can I prune a Tomato plant?

It is probably to late to do any good now. Just give them as long on the vine as you dare before a freeze. Take them in and place in a brown paper bag and they will ripen fine. Those that don't can be made into fried green tomatoes. ;-)

At this point, it's difficult to prune tomatoes without losing the fruits already set on the vines, and also losing the plant's ability to produce energy to ripen them.

Your best bet is to shelter the plants from the frost by at first covering them lightly, then removing the covers in the day when the weather's warmer. You can even set up a couple old glass windows in a tent over top of them to make a mini-greenhouse. Tomatoes are hardier than you'd think, and survive makeshift greenhouse conditions quite well, even under several inches of snow if they're protected.

Later on, though, you may want to pick the remaining fruit and ripen them on a sunny windowledge.

I like the first answer. I do that often and extend my season a month at times in the fall. Quit watering them earlier in the fall. This will stress the tomato and ripen the fruit. I also take in my green tomatoes and wrap them in newspaper, laying them in a box, single layer and put them in the basement where it is cool and dark. Have tomatoes till Christmas sometimes.

My Dad used to pull a few plants out, roots and all before frost and he'd hang them up-side-down on our enclosed back porch or in the basement. The fruit would continue to ripen for a few weeks.