So, in your case you could do something like:

CASE WHEN {yourfield} = '1' THEN 'T' WHEN {account} = '2' THEN 'T' WHEN {account} = '3' THEN 'T' ELSE 'F' END

So, if yourfield is 1, 2, or 3 and someone selects 1, 2, 3, 4, 5, and 6 from the filter region, it will still only bring back results for 1, 2, and 3