When you click the print button on a page it performs a refresh of the page and adds "printer_friendly=1" as a url argument.  When this is performed on a donation thank you page it will result in the donation form returning to the start.  If you would like to remove the print button on the donation thank you page then you can conditionalize the wrapper content to behave differently when the url contains "donation=completed".

Users will be emailed a thank you autoresponder and depending on your site's configuration it could have a pdf receipt attached.  If you do not have receipts enabled and would like to:How do I enable Receipt Manager and upload Receipt templates