Your not considered military until the day you start basic training you can than back out of a contract, if you have already started basic training than you have to serve your term out .
You have to tell your recruiter that you don't want to join so you can go back down to MEPS and DEP Out. If you just ''don't show up'' possible UCMJ violation and prosecution. If you DEP Out, no you won't barred from joining again.