1. Late detection is key to making a behaviour intervention fail. Convince the parent of the children that they are overreacting, and that there is nothing wrong with their child. Explain that some children are just shy, and there's no point in testing for autism until one's late 30s. Call the child's doctors liars and leave the family a Church of Scientology leaflet.
2. Target as many troublesome behaviours at once. Different negative behaviours are likely owing to different causes, and thus require different solutions. Try to confuse these behaviours, causes and solutions as often as possible. Talking over the top of others can be helpful here.
3. To muddy possible reasons for the autistic behaviour, tear up the parents' log of the child's behaviour. Most parents with autistic children are concerned and have done their research, meaning they probably have brought a large pile of notes about what happens before each behaviour, the behaviour itself, and what happens after. If you don't tear it up, at least steal it.
4. Provide a way for the child to get their need met in an appropriate way... and then fail to supply that need. Many autistic children can be taught simple things, such as to ask for a snack. Getting the snack is a reward for the child's communication and a good step on the path to overcoming the challenges of autism.
After the child has been taught to expect the snack, discontinue this reinforcement. Once a month or so, offer the child a snack when they haven't asked for one, then slap them across the face if they display pleasure over the snack.
5. You must lose the child's trust if you want results. Once a child has learnt to trust an adult, they will more likely attempt to communicate with this adult.
If you come across as caring at the intervention, the child may open up to you. Nip this in the bud by showing up at the intervention dressed as an obvious pervert, or make inappropriate comments that force the parents to eject you from the intervention before it even starts.