Remove ALL styling/formatting from hyperlinks

The property-value pair:

a {
    all: unset;
}

seems cleaner in my opinion and has the advantage to work with all the selectors, e.g.:

a, button /* &c... */ {
    all: unset;
}

To learn more about the all shorthand, checkout this page: https://developer.mozilla.org/en-US/docs/Web/CSS/all
As Emi said, don't forget check for compatibility: https://caniuse.com/css-all

You can simply define a style for links, which would override a:hover, a:visited etc.:

a {
  color: blue;
  text-decoration: none; /* no underline */
}

You can also use the inherit value if you want to use attributes from parent styles instead:

body {
  color: blue;
}
a {
  color: inherit; /* blue colors for links too */
  text-decoration: inherit; /* no underline */
}

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.