1) In your menu, click on "Settings".
2) Click on "Users".
3) Using the search bar in the top left, locate the user. Then, click on their name to open their profile.
4) Click on "Cancel Invite." This user will no longer be able to join your Prospr Workspace.
If a user has accepted their invite, instead of canceling an invite you will need to disable the user instead:
How to Disable or Re-enable a User's Account
Change a user's account status