If the program is giving you a sparge step, then you are not using a mash profile which is set for full volume BIAB. Open up your mash profile and click on the box labeled 'BIAB mash with full boil' and this will tell the program to add all of your mash water to the infusion and reset the water to grain ratio based upon the grain weight of the recipe.
Doing this will remove the sparge step from your recipe.