Redirect all GET and HEAD requests to new domain (only redirecting GET and HEAD because POST, PUT, DELETE etc to old domain is almost certainly automated spam - no genuine users should ever be posting to old domain if it's been completely redirected). Serve 403 to any other request.

RewriteCond %{REQUEST_METHOD}  ^(GET|HEAD) [NC] # Only rewrite GET requests
RewriteCond %{HTTP_HOST} olddomain\.com [NC] 
RewriteRule ^(.*)$ http://www.newdomain.com/$1 [L,R=301]
 
RewriteCond %{HTTP_HOST} olddomain\.com [NC] # Otherwise block
RewriteRule ^(.*)$ - [F]