Maybe you aren't firing enough people and do it fast enough.
Lay out your expectations, have a code of conduct and a handbook.
When some one messes up can them. You won't have to fire many this way before you create a culture in your company. You need to be known as a hard@$$ when it comes to conduct and what will get you fired. Those stories of the people that were fired will permeate a company for years and keep the new guys in line.