This tutorial covers the two ways to take access away, and when to use each.
Things to Consider
There are two different removals, and they are not the same thing.
Remove account-level access takes away someone's account role but leaves any project roles they hold. They keep seeing the specific projects they were added to and stop seeing everything else. Use this when a person's remit narrows to one or two jobs.
Remove from account takes them out of the account and its projects entirely.
Steps
Open Settings (gear icon, bottom of the left sidebar) and click Users under Account.
Click the actions menu at the end of their row.
Choose Remove account-level access or Remove from account.
Remove someone from one project only
Open Settings, click All projects, open the project, and use the actions menu on their row in the Users tab. This leaves their access to other projects alone.
If "Remove from account" is greyed out
Some users cannot be removed from the row menu. Start a chat from the message icon in the lower left and we will sort it out.