Awkward/tricky/tough etc customer noun
Used to refer to a person who is difficult to deal with
This morning Nick helped me deal with a tough customer.
You should use the tough treatment for these tricky customers.
Someone who has a strong personality
It is difficult for someone (not) to do something.
To be difficult, troublesome, or unpleasant for someone; to be to someone's disadvantage.
Used to describe the action of treating someone very strictly.
If you say that someone or something is too hot to handle, you mean that they are too difficult or dangerous to cope with.
If you fall for someone's trick the first time then they are at fault.
Are you going to mess with me again? Fool me once, shame on you; fool me twice, shame on me.