To hide a form that you no longer need, you can archive it by clicking on the black box icon to the right of your form's name in the Contribution Forms tab of your Dashboard.
If you choose to archive a form it can no longer be viewed, receive contributions, or be edited. However, you can revive the form if you need to.
After archiving a form it will turn grey (as you can see below). Once you leave the Contribution Forms tab after archiving a form, you won't see it in the Contribution Forms list anymore. But don't worry: You can use the search tool in Contribution Forms to find archived forms. If you want to revive an archived form, click the reset arrow icon where the box icon used to be.