Clear all imported CSS styles?

I need to embed WordPress into my own existing website design. I’ve decided that the best thing for me (despite all the extra work) is to create my own custom theme from scratch. At this point, my biggest problem appears to be conflicts with my existing website CSS. I tried to override certain site-wide CSS by using inline CSS but something must still be conflicting somewhere because there’s a few elements that I just can’t seem to alter. So, does anybody know of any way to clear all previous CSS statements within an element (like a DIV) so that no CSS from the main style sheet would apply to it, and I can then declare all new CSS inline?