I use odorkill, i have four cats and one in paricular is very sensitive to change - he used to urinate next to the front door (obviously as we walk in and out new smells and secents are brought in and made him uncomfortable) i cleaned the area thoroughly with odorkill and although this got rid of the smell and cleaned the area it didnt stop him going back to there again (because of the outside smells) i continued to clean the area with odorkill and bought a Feliway Diffuser. Hey presto at 6yrs old he stopped, the carpet is clean because of the odorkill and he feels happier because of the feliway.
Saved me having to buy an entire new carpet, cat and owner are now happy :)
Odorkill is available from the animal health company (see link above) and i get my feliway from http://www.petdrugsquick.co.uk/?p=siteinfo - its the cheapest that ive found it on the net (much cheaper than the vets!)
Good luck and hope you can stop those bad kittys :D
Roxanne