Hi Stephen --
You can issue a conditional TCP::close (at the end of the response in the 2nd rule) to prevent subsequent requests on that connection, or return / event disable (in the relevant events) to prevent processing of all or specific parts of the subsequent iRules.
Either method of connection control would use the following approach: If some specified condition occurs within the first rule, set a variable as a flag in HTTP_REQUEST in the first rule where you are currently using "return" indicating that the condition was seen, then in the second rule, use the value of that variable to control whether the return, event disable, or close is processed.
HTH
/deb