After disabling the SRSTrust feed via the UI, run cbfeed_scubber to remove the feed's reports. Use the options -v for verbose and -u for untag.
/usr/share/cb/cbfeed_scrubber -uv srstrust
A temporary workaround to block the display of previously tagged events (after the feed is disabled) is to set the cb.conf configuration raising the feed's minimum hit score. The default is 1; Setting it to 100 basically says ignore any SRSTrust tagged events.
FeedHitMinScoreSrsTrust=100
Alternatively, modify the query to include the date the feed was disabled.
alliance_score_srstrust:* AND start:[YYYY-MM-DDT00:00:00 TO *]