iRule to block\reject connections from Lync client

That way will work. The switch statement may make things simpler and faster.

when HTTP_REQUEST {
        switch -glob  [string tolower [HTTP::header "User-Agent"]]
        "microsoft lync*" -
        "microsoft outlook*" {
                log local0.info "Client IP:[IP::client_addr] has been blocked with user agent :[HTTP::header User-Agent]
                drop
        }
        default {
                log local0.debug "Client IP:[IP::client_addr] attempt with user agent :[HTTP::header User-Agent] successful"
        }
}