From the output of 'fbsvcmgr -?' one may see:
. . .
trc_name [string value]
trc_id [numeric value]
. . .
But it seems that we can`t specify key-value pair like 'trc_name <my_trace_session>' in order to STOP previously launched trace.
Following is what I did in cmd.exe having prepared trivial config for trace ('tmp_trace.cfg'):
C:\...> start fbsvcmgr localhost:service_mgr user SYSDBA password masterkey action_trace_start trc_cfg .\tmp_trace.cfg trc_name my_favorite_trace_ssn_01
// OK, new window will be opened with launched trace session
Error: parameter "NAME" is incompatible with action "STOP"
So, either this option should be excluded from output of fbsvcmgr (and this will be really BAD!) or `action_trace_stop` should be pretty clever to stop trace sessions by their names (and I'm strongly voting for 2nd case :)).