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You can usually safely delete a user but you should proceed with cautions and watch the prompts.
Potentially not a disaster – the content will be in the Bin / Trash. You can navigate to the bin and you should be able to restore most of the content. This can be a little messy though so we advise taking a backup of your database first.
To prevent a user making changes and accessing important parts of your dashboard, you can just edit their account and change their role to Subscriber