Yes, if you do not wish to use the Donor Portal you can turn it off. Note that disabling the portal will also disable the peer to peer campaign functions as well as online auctions. 

From the Donor Portal Info page, change the toggle for Disable Portal Access? to YES. Scroll to the bottom of the page and click Save to save the changes. The link to "Create Account" or "Sign In" will no longer be visible on any of your donation, membership, event, or auction pages.