can i back out of lease if i never took possession

I signed a lease online for housing in Los Angeles after putting down a $500 deposit. I cannot live there anymore, however. According to the lease I would take possession August 12, 2011 of the apartment. The lease has been signed by the owner but has not been finalized since I have not had a guarantor sign the lease as required. Can I back out of this lease and let them keep my $500 deposit or am I required to take possession of the apartment. The property management is saying that I would be required to pay rent until they can find someone else to rent the apartment.