Modification in single campaign page

Hi. I need to override the small form that appears on the simple campaign page, at the top right, just below the status bar, which allows you to enter an amount and hit the “back campaigning” button. Instead you would like to insert a fixed text.
What I intend is that only amounts can be donated in rewards.
Where can I find the file in which I can cancel that form and change it to a text?
Or there is another way to achieve the same result, which can only be donated as a reward?
Thank a lot!