irule is unable to block all of the unwanted UDP syslog messages which is not there in the string filter

Please see the below irule

when CLIENT_ACCEPTED {

 UDP::payload replace 0 0 [binary format ssssa256 255 255 255 255 [string repeat "~" 256]]
   if { [class match [UDP::payload] contains syslog_strings] } {
   binary scan [UDP::payload] ssssa256a* a b c d e data
   UDP::payload replace 0 [UDP::payload length] $data
   pool pool1

} else { discard } }

This irule is configured to only allow the UDP messages in the asa_syslog_strings to forward to the pool pool1

Please help ASAP