How to stay away from toxic people in business

Business whether small, medium or large need pool of network. You need to be friend with everyone and foe with none. But in practice you need to enlarge your friendship aka client base and lessen the number of toxic people around you.
No matter how hard you try, while doing business, you will meet people toxic to you.
That one irritating guy could be any, an office colleague, a vendor or supplier or client. Of these a client if toxic done much damage. But you need to welcome every client. And there is no set rule to identify a negative client till you not work together. So remedy is set your rules and stick to them.
Here are 5 points:
1. Listen your client what he / she want. Next ask to put his / her needs in writing.
2. Some clients shows like they are doing charity by asking you for work, they are always over demanding. Say no to such people. A polite no.
3. Tell your client clearly what you can offer and stick to it.
4. Some clients start asking extras midway of a project. Simply turn down the request. Stick to what is promised.
5. Set your rules and make a legal agreement before you deliver the product or a service.