Transactions are created for the updated security prices under the journal 'Unrealized Gains or Losses".
To delete these transactions, go to Transaction Entry for the Category:'Unrealized Gains or Losses' and the Account:'Unrealized Gains or Losses'These transactions can be deleted just like any other transaction.
Since you "cleard the incoreect posting by activating "Changes in Account Balances"', you should run Utilities | Test and Correct | Balances for Accounts. This will correct the account balances by adding up the transaction detail for each account. This will eliminate the "Changes in Account Balances".
Let us know if this does not solve your problem.