Remove Custom Payment from certain campaigns?

Perhaps I have somehow overlooked this, but have not figured out how to remove the Custom Payment field (the blank field at end of preset payments).

I would like to have this ENABLED on some campaigns but not on others. We have an event with sponsorship fees, with preset payments ($50-$500) and I do not want them to be able to choose their own amount. BUT another campaign is contributions to the scholarship fund - it has some preset amounts, but would like them to be ABLE to fill in a custom amount too.

How might we go about doing this?
Thanks.

*Bump*

Hi

If you know php code , you can modify the file components/com_jdonation/views/donation/tmpl/default.php .

Thanks,
Tuan