Attempts to hide/remove these types of pins:

• promoted (sponsored)
• commerce pins (aka product pins, shopping pins, buyable pins)
• ideas for you, ideas you might love
• topics for you
• searches to try
• fresh ideas

Requirements:
Tampermonkey or Violentmonkey on a recent version of Firefox or Chrome.

Greasemonkey does not work properly with this script. Also, Safari is not supported.

Extra notes:

• Removed pins leave behind a "hole".
• This script is not currently configurable.

Known issues:

• When first loading Pinterest, there can be a second or two before this scripts starts hiding things. I hope to fix this in the future, but no luck in my attempts so far.
• I believe only promoted pins are blocked on the search results page. Pinterest changed things around in a recent update, and I haven't got around to making a proper fix yet.

Alternative using uBlock Origin:
If for some reason this script doesn't work, or you just don't want to install it, you can get a similar effect by using uBlock Origin with the following filters (you may need to modify them, especially if you're not using English):

pinterest.*##[data-grid-item],.Collection-Item:has([data-test-id="oneTapPromotedPin"])
pinterest.*##[data-grid-item],.Collection-Item:has([aria-label="Product Pin"])
pinterest.*##[data-grid-item],.Collection-Item:has-text(/ideas you might love/i)
pinterest.*##[data-grid-item],.Collection-Item:has-text(Searches to try)

Note: Cosmetic filtering must be enabled. Also, if you don't know, the above is placed in the "My filters" tab of the dash board.