I guess it depends on what you mean by "exploit" or "take advantage". We have users that have attempted to exploit our systems by way of hacking utilies. They get suspended immediately. Spammers are another that we don't put up with.
Now if you're talking about a user who wants you to install "X" script, it doesn't work, then they want you to resolve it for them - that's something else. We'll gladly help with what we can, but we make it very clear to our customers up front that we are not web developers or programmers for their site. We can possibly assist (given that we've all done HTML design at some point in our life), but really they need to contact their programmers.
If at any time it comes down to an abuse of staff in which users swear at the staff, customers get ONE warning, then suspended and terminated. Much like you can't go into a brick and morter store and scream your head off, we have the same policy with online help.
We underpromise and overdeliver (the 90/110 rule), but there does come a point where no matter what is done, the client (or us) will not be happy. At that point you must cut ties and advise them to seek hosting elsewhere. Often, if pitched right, telling someone to find another host will settle them down and they become one of your better clients.