[MickDoneDee]

Admin CP - Import & Maintenance - Execute SQL Query - Paste query in Manual Query edit box - click Continue

Copy and Paste exactly as below:

SELECT `referrerid`
FROM `user`
WHERE referrerid >0

If you're happy that the correct data has been called then do another query. This time, copy and paste the DELETE version which should call the same data and, hopefully, give you the option of deleting it.

DELETE `referrerid`
FROM `user`
WHERE referrerid >0

Later, you can do another query to determine who received the most referrals.