I don't put up with abuse in real life, I don't think any of my support staff do either. Why should any of us put up with abusive behavior online - even from potential or current customers?
For demanding or abusive shoppers, realize that you don't have to justify every point of your business plan. If they make demands that you aren't going to meet (for whatever reason), just keep refering them to your current listed offerings & your TOS and say that you cannot meet their needs, it's not part of your business plan. You may get some people who change their demands to see if you'll budge - either repeat your previous responses, or (if they reach a reasonable point) make a deal. But don't snark at them, don't say you get what you pay for (they'll either find that out, or make peace with being at an unacceptable host), don't get defensive. It just raises your blood pressure unnecessarily.
For abusive customers, do what corporate HR departments do: first reminder that such conduct is abusive of the staff and won't be tolerated; second reminder and a notice that if this happens again they may be asked to leave; final reminder; review of all prior incidents (documentation is good here - if you can refer to ticket numbers, even better); and a notice of service termination (with or without grace period, though I would recommend 3 days' grace with no shell access to let them get their files off of your system). If you state this policy clearly in your TOS, you're covered legally; if you define the abusive behavior in your reminders to the customer, you're teaching them exactly what not to say or do again. Just stay calm and consistent when dealing with them. There's no need to raise your or your staff's blood pressure; and you just might teach them that while they are a paying customer, you won't take abuse - and you won't give it, either.
If someone is irritated because of a particular problem and that's showing through, we tend to give a bit more leniency. Thus far, it's been possible for us to see when clients are just having problems troubleshooting and are thus carrying on cranky, versus truly abusive people who want to put us on the defensive or make us feel like we owe them something. The folks who are just exasperated and upset get a bit more leeway.