you can't...CR isn't that versatile. The simplest solution to accomplish this is to write a stored procedure, there you can delete a whole group or really do anything that you want.
CR can only read 1 record ahead and 1 record behind, but it can't affect those records.
Another way to accomplish this would be to have a subreport run for each group and do some conditional suppression there as well as having either running totals or formulas that act as running totals / flag so that the totals displayed match the values displayed (obviously, the first solution--stored proc is simpler in the long run)
HTH