How do I deactivate a Portal User for given Contacts in Apex Code?

Here you go!!

Map<Id, Contact> conmap = new Map<Id, Contact>([select id from contact where Account.RecordTypeId='012xxxxxxxxxxxx']);//add filter based on your need
List<User> usersToUpdate = new List<User>();
for(User u : [Select Id, IsActive, IsPortalEnabled from User where ContactId in :conmap.keySet()]){
    if(u.IsActive || u.IsPortalEnabled ){
        u.IsActive = false;
        u.IsPortalEnabled = false;
        usersToUpdate.add(u);
    }
}

if (usersToUpdate.size()>0){
    update usersToUpdate;
}