As Chris said before me, just an a should override. For example:
a { color:red; }
a:hover { color:blue; }
.nav a { color:green; }
In this instance the .nav a would ALWAYS be green, the :hover wouldn't apply to it.
If there's some other rule affecting it, you COULD use !important, but you shouldn't. It's a bad habit to fall into.
.nav a { color:green !important; } /*I'm a bad person and shouldn't use !important */
Then it'll always be green, irrelevant of any other rule.