In the dashboard, go to 'Add Functionality'.
Scroll to the add-on you wish to remove and click the 'Edit' button.
Now click the 'Uninstall Package' button.
If you want to remove the files from the server to free up space or because of a bad install, you can tick the box for 'Yes, remove the package's directory from of the installation directory.'.
Click the 'Unistall Package' button.
Confirm the you want to do this by clicking the 'OK' button on the popup confirmation box.
You have now removed the add-on from the web site, so if you were to try and add a block the add-on you just removed will no longer be available to use.
But you will still see that your add-on is still in the list of 'Downloaded and Ready to Install' on the right hand side of the 'Add Functionality' page.
This is becuase the add-on is associated with your Concrete5 profile on concret5.org.
If you want to have remove the add-on from the 'Downloaded and Ready to Install' list you only need to do the following:
Click the 'project page' above the 'Check for upades >' link (this is still on the 'Add Functionality' page).
Sign-in to your concrete5.org account.
Scroll down to the 'Add-Ons and Themes' section of the page and find your add-on.
Click the 'Release from Project' button.
In some cases where you have uninstalled a project you might find the add-on listed multiple times on your concrete5.org project page. If so, just click 'Release from Project' for each instance of the add-on.
If the add-on you have released is one that you have paid for and are no-longer using on your web site, it will now be available to associate to another project on your account.