If you want to drop a connection by using the L7-filter instead to shape it, you should not use the Classifier in the [QoS]->[Classifier] section, but the FORWARD chain in the [Firewall]->[Manage] section. The interface of the firewall is the same of the QoS Classifier’s interface and hence you will are able to set the l7-filter and select the DROP target.
Regards
Fulvio