iRule not effective for existing connections

thibino is correct that iRule changes only take effect on new connections.

You could leave the iRule enabled at all times, but use a data group to store an on|off value. The iRule would use the class command to check the data group key to see if non-GET requests should be blocked.

https://devcentral.f5.com/wiki/iRules.class.ashx

when HTTP_REQUEST {
    Check if the maintenance key is set to on in the maint_dg datagroup
   if {[class lookup "maintenance" maint_dg] eq "on"}{

       Check if this is not a GET request
      if {not ([HTTP::method] eq "GET")}{

          Send an HTTP response with cache control headers set to not cache the response
         HTTP::respond 200 content {Please retry your request later.  Maintenance!} Connection Close Cache-Control No-Cache Pragma No-Cache
      }
   }
}

Aaron