So, if you want to use the forms authentication (I assume you meant logging into the Kentico admin app) then you can simply mark the users as NOT domain users. When you edit some user, there is a flag telling the system "Is external user" and "Is domain user" so Kentico knows that the user has to be authenticated against external provider or against the AD domain. So, setting the "Is domain user" to false should do the trick for you. You can do it also in the CMS_User table and then restart Kentico app.