Understand how they are being rewarded for their behaviour

If you are dealing with difficult people, think about how their behaviour is working for them. There must be a pay-off or they wouldn’t continue with the bad behaviour.

Help them find better behaviours and reward those instead

Once you have found the pay-off stop rewarding the behaviour with it. Direct your difficult person towards more suitable behaviours and reward those instead.

Be consistent and you will be amazed at how fast they can learn to drop their old strategies for new ones, especially when they are being rewarded for it.