Users can only be deleted if they have no posts. So staff needs to delete the posts too. There is a setting that configures how old those posts are allowed to be and how many posts are deleted. If the user has older posts, this timeframe can be increased by a setting. Also the number of posts that can be deleted can be increased. Deleting a lot of posts can cause gaps in the conversations.
In general, the preferred way is anonymization, which deletes all personal information like username, email address, password, and so on, but not the posts. So the topics where the user participated still make sense.
By default, users can only self-delete their account if they have 1 or less posts (but that can also be increased by admins).
So to delete an account at another forum, you need to ask at that forum. Only staff at that forum can help you.
If you are further interested, there are already some similar discussions: