Open the wishlist's access (sharing) settings, find the person, and either pick a new level for them or remove them. Only you, the owner, can do this.
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. Moving someone up gives them more control. Moving them down, say from Full Access to Contributor, limits what they can touch.
To take away access completely, remove that person. They lose edit access right away.
There's no separate save. The change applies as soon as you make it.
What changes and what doesn't
Removing someone, or dropping them to Viewer, takes away their editing. It doesn't delete the wishes they already added, which stay on the list.
Someone you drop to Viewer can still open the list and reserve a gift, if you allow reservations. To stop them seeing it at all, remove them and check the wishlist's visibility. See "What is the difference between viewing and editing access?"
Good to know
A pending invite nobody accepted is removed the same way, which cancels it.
Changing someone's level doesn't email them about it. So if a co-planner is going to be surprised they can't edit any more, tell them yourself.
Only the owner can add, change, or remove access. Someone with Full Access can see who has access but can't manage that list of people.
This is also the tidy-up after an event: drop your co-planners to Viewer, or remove them, once it's over.
Related
"What's the difference between Full access and Contributor?"