Is there a way to "clear" a CSS property as if never existed?

Is there a way to "clear" a CSS property as if never existed?

I am using a CDN css file which sets a "top" property for an item which
was recently added as a new release. This 'top' property completely throws
off the height of a list item in my code. I am certain this is the culprit
by use of Firebug.
Normally, I am able to override previously directed CSS properties (such
as height, color, etc) but is there a way to essentially say "forget that
I told you to set top: 24px, I want you to ignore that".
In essence:
.some-class > a:after {
....
top: 24px;
}
(in another file)
.some-class > a:after {
top: gothehellaway
}
Note: I have tried setting to 0, auto, and inherit without successful
results.