wilderness - 3:44 pm on Feb 25, 2012 (gmt 0)
are you using the closing line of:
RewriteRule . 403.php [L]
Or and as lucy suggested:
RewriteRule . - [F]
There are certainly other options.
FWIW, I never used custom 403's until my recent ractivated website.
I've always believed that a plain-Jane 403 served by the visitors own machine is most appropriate, however every webmaster has their own preference.
In addition, many of the shared hosts will automatically insert a custom 403 with advertising for their services, if the webmaster doesn't have his/her own custom 403 in place.