I will add a web.config setting for the next release of mojoPortal so you will be able to add this in user.config:
<add key="DisableAutoCompleteOnLogin" value="true"/>
You can go ahead and add it now so it will be in place after the next upgrade. The default setting will remain false.
Then to solve the problem immediately, you can edit the file /Controls/LoginControl.ascx
Look for the <asp:TextBox id="UserName" ... and <asp:TextBox id="Password"
add the attribute AutoCompleteType="disabled" on both of them.
That should do it.
However note that older web browsers before html 5 do not support this attribute except for IE, it was a proprietary IE feature until html 5. So it should work in modern browsers but may not work in older non IE browsers.
Hope that helps,