Hi Alberto,
In your question you asked, how to disable the "Compress Prepayment" by default. By Default means by setup or Page setting without the customization. Hence i told you it is not possible by default. You have not at all mentioned that how you can achieve this through customization / Change in code or property.
Then you mentioned that it is possible through the customization.
If you ask how you can achieve this by customization, then there are multiple ways Like apart from the solution you mentioned-
1. You can disable the "Compress Prepayment" option while opening of Sales Order Page, by writing the code on Page Trigger.
2. You can also disable the "Compress Prepayment" while entering the "Prepayment %" by writing the code on "OnValidate" trigger of "Prepayment %" through table extension. So the "Compress Prepayment" will disable only in case users enters the "Prepayment %".
I think i have not understood your question correctly or may be i misunderstood from your statement (By Default). Good to hear that you have solution now.