I have recently signed a year long lease with a roommate who seemed very nice at first, though over the course of the last two months, she has been violating our lease by letting her boyfriend stay constantly. She has given him a key without my permission and I do not know him at all. I have tried to explain how uncomfortable I am, but it does not seem to phase her.
We recently had an argument over this and she has decided to leave. To prevent her from leaving without paying her bills I tried to read up on SC laws, however I'm not sure which scenario to use. I assume she has to give me 30 days and pay this month because of the eviction laws by a month-to-month basis.

Are there any "roommate" laws or something that can protect me better?

I am not aware of any "roommate" laws, in SC or anywhere else for that matter. Your roommate is treated as a sub-tenant. In other words, you are the landlord, an the roommate is considered your tenant. If the terms of the lease have been violated you can evict the roommate in the same way that your landlord would evict you for violation of your lease.

Its not clear whether you are the owner/leasee or you are the renter. If this person moved in with you, meaning you own the property or lease it from the owner, then she is your tenant.

There are no separate laws on this and you would evict her for breach. You may be able to give her a 3-10 day pay or quit since she has breached the lease.

It's not clear that there was a violation of law, regardless of what the lease says. A guest, 'constant' or not, is not grounds for eviction. If you were in court and the boyfriend showed that he had a residence elsewhere, you would probably lose. Landlords are often shocked to find out that they can't even evict for extra tenants they didn't know about, although such laws don't always apply to 2 or 3 unit owner occupied buildings.

If you had a side written contract with her that specifically defined guests, you may find a sympathetic small claims judge, who rules that your roomie owes for 30 days past her notice to leave. But given that the law strictly speaking isn't on your side, your roomie may argue that you effectively forced her out, and a judge might rule against you. I'm not sure you should expect payment past the day she moves out.