You can't remove posted transactions.  You have to process a transaction that "pays" that finance charge for the client.  I suggest deciding what GL Account should be offset.  Probably the original Finance Charge GL Account.  Then do a zero dollar cash receipt.  Enter as the Check Number something like "WriteOff FC"; then the amount received from the ,customer is $ 0.00.  On the lines tab; pay that FC Invoice so the balance is $ 0.00.  Do a GL Posting on the next line to post to the GL Account you decided to use.  (Meaning, after the FC Invoice is paid, your transaction is out of balance.  Go to the second line, switch from Invoice Type to GL Posting, and enter the GL Account.  Post the dollars getting you back to an in balance transaction.  Go on to the next transaction.  Carefully look at the register and the GL Posting to confirm before answering yes to Update.  

To reverse a finance charge, I use the Finance Charge Entry and make a negative entry. Post it, and apply the positive and negative entries. This will also update the Statistics tab of Unpaid Finance Charges.