iRule to block multiple user agents.

Is using a string type data group an option for you? Something like this:

    ltm data-group internal blocked_user_agents {
    records {
        androiddownloadmanager { }
        baiduspider { }
        grapeshotvrawler { }
    }
    type string

    when HTTP_REQUEST {
        if { [class [string tolower [HTTP::header "User-Agent"]] contains $blocked_user_agents ] } {
            drop
            return 
        }
    }