You can convert a User that is linked to a User Sync into a local Cheqroom user.
Turning the user into a local user
In the Settings screen, go to Users
Find the user you want to change
Click the Actions button
Click Clear sync
Next, click Reset password
The user will get an email with a temporary password; they can choose to change it later
Updating your User Sync
You'll still need to ensure that your User Sync will keep that User a local user instead of turning it back into a Synced user automatically at the next nightly run.
Set Overwrite local users to OFF