How to reject grace? Let us count the ways.
Blaspheming the Holy Spirit, for one; deciding on what is good or evil for ourselves or others without reference to God; creating one's own church.
Basically choosing hell over heaven.
St. Paul (1 Cor 3:12) says that it is up to every man to accept grace or to reject grace. That choice must be made every second of every minute of every day. It cannot be a one-off event. It must be continuous - to love God with your whole heart, your whole soul and your whole strength. One single event is enough to reject grace and you must take steps to regain it or else you remain in a state of sin or in a state without grace.
1Cr 1:18 For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God.
2Ti 1:9 Who hath saved us, and called [us] with an holy calling, not according to our works, but according to his own purpose and grace, which was given us in Christ Jesus before the world began,
Tts 3:5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost;