You have 120 days from dispersement return unwanted loan money wihtout pentalty (i.e. they subtract it from your original principal and credit any origination fees). Since it's most likely that you got your money more than 120 days ago, your best bet is to return next year's money, or reduce that amount taken (if it's not too late to just change your award for next year). Turning down aid money will NOT affect your award for next year.
The subsidized perkins loan cap is $8500/2 semesters. However, this number is also affected by your EFC. If you recieved unsubsidized loans, you surpassed the maximum amount of subsidized loans that were available to you for that year. Hence, you can not turn unsub loans into sub loans magaically. If none of the above options work for you, you can by all means write direct loans a check with your abundant wealth. You will not be charged a penalty fee, however the origination fee will not be deducted either. It will just be like you made an early payment on your loans.
However, I'd give serious thought before returning the extra money especially because of today's interest rates. The unsub deferral rate is something rediculous like 3%. I'm going to guess that you're an M1. It's not a bad idea to have some extra money in the next couple of years escpecially if you need to finance clinic-appropriate dress, a car, a new computer, away rotations, traveling to interviews, relocating, etc. As low as the interest rates are, in mot cases it's certianly worth holding onto a couple extra grand (even $10k) before you have a better idea of what your living expenses will be like for M3, M4. Unsub Perkins loan money is certainly at a much better rate than relocation loans available at the end of M4.