I've looked and not found anything and right clicking on it doesn't bring up a context menu. Thanks.

Close Scratch and re-open.

Oh, are they only per session? That's pretty inconvenient for those who use them every time, but great for me in this case, thanks.

Actually, stevedawson is wrong. They stay with your Scratch project. As far as I know there is no way to remove them. I'll dig deeper and figure this out for you.

That would be great, thanks!

If there are no blocks used that are associated with that extension, the next time you open the project it will be gone.
Hope this helps!

If there are no blocks in any script for the extension, it will be gone the next time you open the project

Thanks!

Thanks LegoManiac04 and IsaiaContreras.That was really helpful

Cool games guys!!!

Ik! Don't use a block of it! And get out of the project and go to your profile, then open it back up. (It will be gone.)

clear

Just pretend this extension block is here. And next time you open it, its gone.


(MAC COMPUTER ADDRESS)
My browser / operating system: MacOS Macintosh X 10.13.6, Chrome 72.0.3626.121, No Flash version detected

First, remove all the blocks in that extension. I'm going to use pen for this example and this only works for Scratch v3.0 (for Scratch 2.0 , just click on the text for the extension and click “remove extension”).
change from

when green flag clicked
forever
if <key [space] pressed?> then 
set pen color to [#ea0015]
pen down
wait (2) secs
pen up
else
clear
end

to

when green flag clicked
forever
if <key [space] pressed?> then 
wait (2) secs
else
end

Second, reload your project (page), the extension should disappear as if the program thinks it's useless because you didn't use any of the blocks.

Closed. Please check the date and read the thread to make sure it hasn't already been resolved before posting. Thanks!