Click on your avatar in the top right of the page, then click Site Admin.

Click Overview on the left. You will find this under the Users heading.

Find the member you want to take action on, then click on the tick box to the left of their row.

Click Bulk Actions... then click on the action you want to take.
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​To suspend a member, click Freeze. They will no longer be able to use the site or log in, but their account will not be deleted. You can allow them access again in the future.
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​To delete a member, click Delete. Their account and all information associated with it will be permanently deleted. This is not undoable.