To disable this behavior, open up admin\Security\Signin.ascx.vb and search for the following lines:
'Override the redirected page title
Dim myPage As DotNetNuke.Framework.CDefault
myPage = CType(Me.Page, CDefault)
If myPage.PortalSettings.LoginTabId = Me.TabId Or myPage.PortalSettings.LoginTabId = -1 Then
myPage.Title = Localization.GetString("ControlTitle_login", Me.LocalResourceFile)
End If
You can either comment them out, or modify the behavior of the if statement.
The alternative is to make a hidden login page. That will set the LoginTabId to be something other than -1, and the "User Log In" will not show up on your homepage.