How to Drive Away Thoughts
No beginner can ever drive away a thought if God does not drive it away. Only the strong are capable of struggling with them and banishing them. But even they do not achieve this by themselves, but with God's help rise up to wrestle with them, armed with his weapons. So, when thoughts come, call to our Lord Jesus, often and patiently, and they will retreat; for they cannot bear the warmth of the heart produced by prayer, and flee as if scorched by fire. John of the Ladder tells us to flog our foes with the name of Jesus; for our God is fire, devouring evil. The Lord is quick to help, and will speedily revenge those who wholeheartedly call to Him day and night.
ST. GREGORY OF SINAI Philokalia, Instructions to Hesychasts