next to the manager, choose Revoke access and confirm. They lose access immediately and are notified by email. The only owner has no ... menu: a project always keeps its owner.
SyncNow ends their membership and removes the project from their SyncNow. The hosting and database connections belong to the project, not to the person, so they stay as they are for you and the other managers. Membership is checked on every request, so their agents can no longer read or change the project; your website is not touched.
You can invite them again later, and their name stays on the work they did in the project's Activity. An invitation that was not accepted yet is withdrawn with Revoke invite, under Pending invitations, instead.