Whoa. Stop. Think about what you're doing! You're trying to change the domain SBS is a part of. This is very very bad news. Most things in SBS depend on the domain they were installed to (Exchange, IIS, Sharepoint, Etc.). When you change that domain, you break just about everything. You also may be violating the SBS license here because SBS cannot hold all FSMO roles in an Open Directory setup (which is a requirement of SBS. A requirement which, if not met, leads to the server shutting down every 2 hours). Please rethink this, Mr. Collins.
As for removing the Certificate Authority service, that can be done through Server Manager's features tab, but it will break things. Also, you won't be able to change the domain without first dcpromo-ing it down. I have warned you though, you do this and you break everything, possibly the license agreement included.
A better idea is to join your Mac infrastructure to Active Directory or even have 2 directories, one for mac, the other for PC. This would be preferred over losing AD goodness and damaging your server to this extent.