My name is***** and I am sorry to hear that your kitties are house-soiling. Do you know if it is ALL of the cats that are house soiling or is it just one of them?
Once we determine WHO is the culprit, we need to determine why?
Have you taken the offender to the vet for urinalysis to make sure they don't have a bladder infection?
Have you considered that there is one cat that is guarding the litter boxes and not allowing the others to use the box?
I would start with the following things-
1) have all of your litter boxes in different rooms. This will prevent one cat from guarding all of the boxes. Also add one more box for 7 today.
2) remove the lids from any boxes, many cats don't like the privacy, they would prefer to see everything while they are toileting.
3) try different litters in different boxes, clay, clumping, shredded corn cob, etc.
Next I would try to determine if there is only one culprit or multiples? If there is only one, I would recommend taking that one to the vet for a urinalysis and possibly blood work.
Lastly, I would try a urinary diet for the kitties such as Royal Canin SO or Calm (that is really what it is called). There are additives that will help reduce bladder inflammation as well as calming herbs to minimize stress that is inducing the cats to house soil.
The following is a link to an extensive discussion on house soiling as well.
I hope this answer helps you. Feel free to contact me if you have any further questions.
Kansas State University
College of Veterinary Medicine Class of 2001