I'm not an SQL expert, so make sure to test this code before using it (tip: replace DELETE with SELECT * to test). Make sure to replace the indicated sections with the correct table and user information. And always make a backup before messing with your database!
Deletes posts for a specific user:
DELETE FROM <YOUR WORDPRESS POSTS TABLE> p, <YOUR WORDPRESS USERS TABLE> u WHERE u.ID = <USER ID NUMBER YOU WANT TO REMOVE>
Deletes post meta for a specific user:
DELETE FROM <YOUR WORDPRESS POSTMETA TABLE> p, <YOUR WORDPRESS USERS TABLE> u WHERE u.ID = <USER ID NUMBER YOU WANT TO REMOVE>
Deletes the user's account:
DELETE FROM <YOUR WORDPRESS USERS TABLE> WHERE ID = <USER ID NUMBER YOU WANT TO REMOVE>
Deletes the user's account meta:
DELETE FROM <YOUR WORDPRESS USERMETA TABLE> WHERE user_id = <USER ID NUMBER YOU WANT TO REMOVE>
Deletes any comments from that user:
DELETE FROM <YOUR WORDPRESS COMMENTS TABLE> where user_id = <USER ID NUMBER YOU WANT TO REMOVE>
Hope that helps!