All our themes are released under the GPLv3 license, which means you can use (and abuse) them in any way you see fit, including adding, editing or removing parts of them, including the footer link.
There is the side-effect, however, of us not providing free support if our credit link is removed. The commercial Plus edition of the theme does not come with such a link displayed by default (and has the built-in option to easily set your own text instead).