Hi there
can i block voip applicaiton using L7. sip now using random port we cannot block by port 
you can read about L7 rules here:
http://wiki.mikrotik.com/wiki/L7
this should work for SIP:
http://l7-filter.sourceforge.net/layer7-protocols/protocols/sip.pat
some others:
http://protocolinfo.org/wiki/List_of_VoIP_protocols