How can I keep my dog and cats away from a certain area in the house that they now seem to want to use as their toileting spot?
I suggest using a Repellent or Deterrent to keep them away from that specific area. There are a few different ones available.
If they have been toileting in the area you need to thoroughly clean the area and remove all traces of the urine smell otherwise they will continue to be drawn to that area. To do this you should use a good quality enzyme based urine cleaner. vet-n-pet DIRECT have a wide range of odour and stain removers available here.