I added too much ground black pepper to my soup (was actually following a recipe for once, while I usually season to taste) - is there any way to rectify this other than diluting it? I think I vaguely remember hearing once that adding lemon juice "removes" some of the pepper...?
A little lemon juice plus a bit of sugar might help. That is a fairly well known folk-remedy. If you can squeeze in something starchy (some rice maybe, or noodles?) that might help too.
A technique that sometimes works for me is to skim off any oil that is on the top and discard it. Piperine (the "active ingredient" in pepper), like Capsaicin (the "active ingredient" in chillis) is oil soluble, and oil floats on water. There is generally a higher concentration of hot stuff in the oil, so removing it can sometimes reduce the heat. While this works a bit in things like curries and chili which contain plenty of fat to rise to the top your soup may not have enough oil to skim. There are other flavor compounds that are oil soluble so skimming the oil may remove some flavor that you'd rather keep in, but if it's just too hot it's worth a try.
Alternatively as you say you could add other strong flavors that can compete with it. Adding acidity will kick off some other taste buds and give your brain other flavors to process, as will sugar. Whether adding acidity, sugar, or other ingredients will work with your recipe is another story. If it tastes good otherwise maybe just live with it if skimming doesn't help.
When I added too much pepper to my soup, my fix was I used the water from boiling the noodles & added a can of petite diced tomatoes. It worked!