Prepaid wireless seller TracFone Wireless Inc. has launched an aggressive litigation tack to block bulk purchases of its cellphones because the phones vanish overseas where the software is unlocked, allowing users to sign up for non-TracFone air time.

TracFone, the nation’s largest independent seller of pay-as-you-go cellphone service, makes its profit by selling the prepaid minutes used by phone customers on TracFone’s system — not from its cellphones, which it sells below cost.