Can I reset a CSS property rather than overriding it?

Since address:not(:last-child) and similars has 11 points of specificity, you can duplicate the classname to make it stronger. For example, the following declaration is totally valid and it has 20 points of specificity:

.mainCopy.mainCopy {
     margin-bottom: 0;
}

And you must add only one mainCopy in your html:

<ul class="mainCopy">

Edit

Take care on the “points” of specificity, because there aren’t decimal points. Them are number positions by the specificity. For example:

 address:not(:last-child) /* is 0-0-1-1 specificity (1 tagname, 1 pseudoclass) */
 .mainCopy.mainCopy /* is 0-0-2-0 specificity (2 classnames) */