You can let them in after 6 months if they complete 1000 hours in 6 months, but you can't keep them out if they complete 1000 hours in more than 6 but within 12 months.
What is their reason? If you are not their employee, they should pay your business- whether that business is a sole proprietorship, corporation etc. You complete the W-9 and they pay based on the information provided on the W-9. Tell them you are registered with the IRS as an S-Corp, and therefore payment for work should be made to the S-Corp (unless your tax advisor says otherwise. Check with you tax advisor as this is a tax issue. ).
Often though, the "plan B" eligibility provision, the 1000 hours in 12 months, requires the full 12 months to elapse like a traditional Year of Service requirement, so it's possible the guy who hits 1000 hours in month 8 doesn't come in until after the full 12 months elapse. So as CBZ said, definitely check what the doc says to that effect.