I know this is an old post but for the help of others I can give the correct solution in case it happens to others. The MYOB support suggestion is incorrect because, that procedure only applies to auto allocated matching of categories. It is totally useless and does not apply to this issue where Conan99 clearly states that on this occasion he manually allocated the transaction category incorrectly!
In this case there is no obvious means to set the transactions category back to unassigned. Attempting to erase the current entry in the Match or Categorise field simple re-enters the incorrect category or the first item in ones category list. In my case that is 6-1000 accounting fees. This is infuriating and really the path to fix it should be much clearer.
The Fix:
To remove a category that has manually been assigned incorrectly to a transaction, you need to delete the corresponding Spend Money transaction that MYOB created when a transaction is manually categorised. This does not delete the corresponding Bank transaction. So don't worry the bank feed item will still be there. Banking > Find Transaction. In the list look for and click on the four digit Spend Money Transaction number corresponding to the offending transaction. Once the Spend Money transaction opens, hit the delete button and acknowledge the "This can't be undone, or recovered later." delete warning. Then go back to Banking > Bank Transactions and you will see the transaction is reset back and is now uncategorised with "Allocate Me" in the Match or categorise column again. Then just allocated the transaction to the correct category as you normally would. Voila.
