When a fundraising project reaches its end date, all recurring donations are automatically stopped and the donors responsible for them notified. Supporters can, however, still make one-off donations to a finished project.
To manually and permanently close an active project to prevent any future donations, login to your fundraiser account, then go to "Fundraiser projects" and click on "Manage > Archive" next to the project name. All recurring donations will be stopped and the donate button disabled for this project.