@dtech the pragmatic answer is that you need to write the software so that the clients that are compliant reject unauthorized network access. Regardless of licensing, the software is insecure if you do not do this.
Remember that GPL doesn't authorize users to do illegal things, so if they break into your systems to victimize your users those users can sue for damages. So you just need to ensure that the compliant software doesn't authorize the things you don't want it to.
Discussion on answer by apsillers: Do…
Discussion on answer by apsillers: Do…