Typically if I go to a spot on a busier stream, I'll just go to another spot on the stream. I don't take my fishing time for granted. On some trips I've been on where we are fishing big name streams, you are bound to run into people in "your secret spot". Learning to fish new spots, and different types of water whether by choice or not makes you a better fisherman.
What do you do when someone knowingly cuts you off, especially on a small stream?
For a small stream, that can be a day ender or pivot to a different creek. If I am fishing up stream, I might go back down using streamers, conditions provided.
I used to get upset by people either low holing or high holing me, on some streams it happens more often than not. I've realized that not everyone is using the same tactic, fly, tippet size, cast, etc, so if you know what your doing, you can catch fish in highly pressured areas more often than not.
This past fall I really wanted to fish a specific spot, so I fished downstream to it, and found guys fishing it with streamers and nymphs. I skipped the spot and went down a ways. I went back to that same spot maybe 30 minutes later on my hike back and pulled a 21" wild brown out nymphing, those guys happened to be walking by at the same exact time as I fighting it. They didn't seem too thrilled haha.