The statement layout will need to be customised. This can be done by your Partner or a Support Staff member.
Add a Print When statement to the paymsg field on the statement. The Print When expression is: ddaccept = .F.
Copy this field and paste it on top of the original one, type in the message you want (general tab/expression) eg "Please don't pay this statement. Payment will be made by Direct Debit etc", then change the Print When expression on this field to: ddaccept = .T.
This means when that tick box is selected they will get # 2 and if it is not, they will get the original paymsg.