This is actually a food myth that has been proven to be incorrect. For a good read check out What Einstein Told His Cook by Robert L. Wolke.
HERE is an article which shows the scientific experiment that he conducted and the results he found. Very interesting stuff.
With salt, and I would say pepper or other spices as well, for the most part, the best way to reduce the amount without affecting the integrity of the dish is to make up another batch without the offending ingredient or at least a lot less of it. Then combine the two.
GB is correct. Dilute the soup, or turn it into a stew, letting the added starch help mask the pepper. Or, make the soup a pepper flavored soup, like pepper-pot-soup.
Besides what's been already mentioned, cooking it longer might help. Pepper has volatile compounds that are driven off with prolonged cooking - which is why it's very important to adjust the seasoning at the end.
The veggies will go mushy and you'll lose other flavors, but the pepper note will be less pronounced.