Yes! You will need the Financial Settings permission to perform this action.
Navigate to "Contributions > Settings" in the left hand navigation and then click on the "Funds" tab. Next, hover over the "Giving Portal Settings" and click on "edit section" button. A pop up will appear.
Under the "General" section, you can toggle the "Enabled?" field.
If yes, the giving portal will be active. If no, no one will be able to give online. This does NOT affect text giving or recurring schedules. The following message will appear when someone visits the giving portal.
Click on "Save" to save your changes.