By default the entries in the Cheque # column comprise a fixed list you can add,edit and delete from plus the most recent 20 entries from the account. You cannot edit the entries from the account because to do so would mean you are changing those transactions.
So the choices are to:
1. Allow what is present
2. Disable picking up suggestions from the account and only use the pre-defined list that you manage the entries in.
3. Perhaps reduce the number of entries it picks up from the account.
To work with the options there are a number of ways, to examine the options you can place the field in edit mode in one of your transactions as you have seem to have done and scroll down the bottom to the Edit List option, selecting that will allow you to work with the various options.
The following Knowledge Base article may also help you: