I marvel at those who charge us in the Free Grace camp of teaching that obedience is optional in the Christian life. That is a ridiculous charge. We strongly and repeatedly teach that God commands us to obey Him and that many blessings come with obedience. Conclusion At the bottom of this question is a fear that those who know with certainty that they are eternally secure will go out and live like the devil. However, that is just not so.
Grace permits the possibility of abuse, yes.
However, grace in no way encourages disobedience. A believer who walks in darkness will stumble and fall and hurt himself every time. There is no such thing as sinning with impunity. Sin never pays. In addition, grace promotes obedience because of the gratitude it engenders.