How to Disable Autocomplete in Apple Safari

Click the 'Safari' menu, and scroll to 'Preferences.'
Select 'AutoFill.'
Deselect the check boxes next to 'Using info from my Address Book card,' 'User names and passwords' and 'Other forms.' This will disable all the sources AutoFill uses to complete forms, so Safari will not have any data to put into a form automatically.
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How to Use Autofill in a Mac

Configure your Autofill settings. To do so, click 'Safari' on the dock to launch the web browser and then click the 'Safari' menu located on the top of the browser window. Click 'Preferences' to launch the browser preferences.
Click the 'AutoFill' tab located in the browser preferences, and then click the 'Using info from my address book' check-box. Click the 'Edit' button to launch the address book, if you want to edit your address book content. Click the 'Edit' button again, and then type your phone number, email and home address. When you finish close the address book.
Click the 'User names and passwords' check-box if you want your browser to remember the user names and passwords you enter on webpages. Click the 'Other forms' check-box if you want your web browser to remember information you enter into forms on webpages. When you finish configuring your Autofill settings, close the browser preferences.
Go to the webpage containing the form that you want to fill. As soon as you begin typing your information in the form, your web browser fills in all the information automatically.
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How to Clear an AutoFill on a Macbook (4 Steps)

Launch Safari from your desktop and click the 'Safari' button located at the top of your browser. Scroll down the pop-up menu and click 'Reset Safari' to launch the 'Reset Safari' window.
Uncheck all the options except 'Remove other AutoFill form text.' Click 'Reset.' Safari deletes all the AutoFill information you have used on your browser.
Turn off AutoFill if you no longer want to use it. To do so, click the 'Safari' button on your web browser and click 'Preferences.' Click the 'AutoFill' button. You should see 'AutoFill Web Forms.' Uncheck the following options to disable AutoFill: 'Using info from my address book card,' 'User names and passwords,' and 'Other forms.'
Close the browser preferences window. Safari will no longer automatically fill in forms.
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