Blocking SQL Injection with htaccess

I've had a recent spate of SQL injection attempts on a site I maintain. The site passes SQL parameters which greatly reduces the risk of a hacker doing anything nasty (look up cfqueryparam if you want to know more); however, the server is still having to process the request and throw an error which has an overhead and also there may be an unprotected query (such as an order by clause) so I've like to add a set of rules to my htaccess file to stop the request at the webserver level (using Apache's mod_rewrite or Helicon's ISAPI_Rewrite on IIS) before it reaches my CFML server.

These are the rules I'm using. I thought I'd share in case it's useful for others and also to ask if anyone has any improvements.

RewriteEngine On

# --------------------------------------------------------------------
# SQL Injection Protection 
# --------------------------------------------------------------------

RewriteRule ^.*EXEC\(@.*$        - [R=404,L,NC]
RewriteRule ^.*CAST\(.*$         - [R=404,L,NC] 
RewriteRule ^.*DECLARE.*$        - [R=404,L,NC]  
RewriteRule ^.*DECLARE%20.*$     - [R=404,L,NC]
RewriteRule ^.*NVARCHAR.*$       - [R=404,L,NC]  
RewriteRule ^.*sp_password.*$    - [R=404,L,NC]
RewriteRule ^.*%20xp_.*$         - [R=404,L,NC]