Hi dlsweb,
If it's for the CMS, you can't change the logoff url. You'd need to write a custom plugin.
If it's for Website Membership, you can set the logoff url in the top of the plugin here:
// After logoff, user gets redirected to the last page they were on (if defined), the url below, or to /
$GLOBALS['WEBSITE_LOGIN_POST_LOGOFF_URL'] = '/';
Hope that helps!
Dave Edis - Senior Developer
interactivetools.com