I now have two solutions that work.
1. Easy one that nobody mentioned: install a sprinkler head on top of the riser, and turn the adjustment screw fully clockwise. This effectively shuts the sprinkler head off. No cost involved.
2. Dig up the space around the riser and remove the riser. THEN the caps that responders were telling me to get will work. Those "caps" will not install on top of the riser. Not knowing what I was doing, I was asking for a cap that would accomplish method 1 above. Not available.
I ended up getting my installation contractor out to cure some other problems I was having, and he told me how to do the above. I did solution 1 and it did work, but since he was there anyway, he did the more professional job, solution 2, for me.[
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Thanks for all the responses. This one is now ready for the archive.