I occasionally use Nextdoor to list items for sale, or to see what's going on in my neighborhood and surrounding area. I have to say, I like Nextdoor since it allows me to connect with my neighbors in a relatively locked-down place, even though I'm sure most of them are in local Facebook groups already.
However, I don't like seeing sponsored content. Period.
Here's how I managed to hide sponsored posts on Nextdoor.
If you have a Pi-hole set up on your network, go ahead and add the following to your Blacklist:
I added it as an "exact" filter and it seems to work fine.
Using uBlock Origin
You can create a custom filter in uBlock Origin that blocks the following domain:
Add the above to the end of your filters on "My filters" in uBlock settings.
I expect this to break in the future as Nextdoor catches on (or finds this post!) and as ad-serving becomes more sophisticated.