Open the wishlist's access (sharing) settings, find the person in the list, and either change their access level or remove them. Only you, the owner, can change or remove someone's access.
Steps
Open the wishlist and go to its access (sharing) settings, where everyone with access is listed.
Find the person you want to change.
To change their level, pick a new one — Full Access, Contributor, or Viewer:
Move someone up (for example, Contributor to Full Access) to give them more control.
Move someone down (for example, Full Access to Contributor, or to Viewer) to limit them.
To remove access entirely, choose to remove that person. They lose edit access right away.
The change takes effect immediately — there's no separate "save."
What changes and what doesn't
Removing someone, or dropping them to Viewer, takes away their ability to edit. It does not delete the wishes they already added — those stay on the list.
Dropping someone to Viewer still lets them view the list (and reserve a gift, if you allow reservations). To stop them seeing it at all, remove them and consider the wishlist's visibility — see "What is the difference between viewing and editing access?"
A pending invite that was never accepted can be removed the same way, which cancels it. Changing someone's level doesn't email them an alert, either — so if a collaborator is suddenly surprised they can't edit, let them know directly.
Only you, the owner, can change or remove someone's access. Someone with Full Access can see the list of people who have access, but can't add, change, or remove them. For a refresher on what each level can do, see "What's the difference between Full access and Contributor?" This is also how you tidy up a list after an event — drop co-planners to Viewer, or remove them, once you're done.