You should right click on the signature in the report footer->format field->suppress->x+2 and enter:
{INVESTIGATION.X_OOS_8_INVSIG} = "F"
The reason I was asking you to put in the detail section is that in the report footer, this field will contain the value of the last detail in the report or in the last group, if there is one. If the value is the same in all details (which would seem to make sense in this case), then the above formula should work fine.
-LB