Are they incompetent, lazy or ignorant?
My experience with this kind of thing is that it starts with leadership, and precedence. Precedence, meaning that people figure out where the line is drawn, and they hang out there. One example I can think of was a new programmer coming on to a job that had been filled by the same guy for 12 years. The code that he had to maintain was a sloppy mess. So he knew he could get away with murder...since the last guy left on his own accord, never getting fired after 12 years of blazing incompetence.
If you've already drawn the line too far out, reign it in. Fire one of em explicitly for not following standards.
It sounds like you've tried to cure the ignorant, leaving you only with the lazy and incompetent...neither of which are worth curing. I'd cull the herd if I were you. If that doesn't work, look for a job outside of management.
EDIT: In response to your "pressurized" addition, the project manager should do their best to absorb as much of that pressure as possible. The best managers know what their team is capable of, and know how to tell management this. You need to be the liaison between team and management, and if management is rushing things along, you should make it clear to them that quality will suffer. If the people with money don't care as much about quality as they do about getting it done...well, you can't be picky about standards. If they want quality, they will listen to a confident manager who tells them that his programmers cannot create a quality, maintainable application without standards enforcement.
If them feeling pressure is what is doing it though, then you need to work on absorbing more of that pressure yourself and not letting it get to your team. That is a big part of being a project manager.
