If you can't shut off the house main you will have to buy or borrow a curb key and shut the water off out in the street. The shut off should be on the sidewalk or in the road.

Then you can replace the house main valve.

It is also possible that there is another shut off valve on the other side of your pressure reducing valve. Take a look.

Sometimes, when a valve is stuck open, it is easier to leave it alone and install a new valve in the line somewhere near it. If you are not confident in sweating in a new valve you can use a compression fitting one or a Sharkbite.

Note to all Mudcatters: All of your faucets, stop cocks and main valves should be operated at least once a year. That will keep them in working order.