found a possible solution to this problem, and yes i know this post is months old, but I've got a memory like an elephant and stuff like this niggles at me...
so, you've got your line with what you want it to do here -
[4] = {name="*** FOCUS ***", search="You gain Focus.", r=256/256, g=256/256, b=256/256, ismsg=1, soundwave="Interface\\AddOns\\sct\\ding.wav"},
then create a line for the event that you DON'T want it to trigger on, but leave the 'name=' field blank, like this -
[5] = {name="", search="You gain Focused Casting", r=0/256, g=0/256, b=0/256},
I can't test it because I don't have a suitable character, but it should work... if someone could let me know that'd be great, then I can lay this niggle to rest