As it was, I saw only greedy and unworthy Quirrell attempting to steal the stone and, I admit, I did all I could to thwart him.
What if your back-up has been thwarted by my guard?
He had been thwarted, both in his attempts to find out what Malfoy was doing, and in his efforts to start a conversation with Slughorn that might lead, somehow, to Slughorn hand-ing over the memory he had apparently suppressed for decades.
In spite of all the temptation you have endured, all the suffering, you remain pure of heart, just as pure as you were at the age of eleven, when you stared into a mir-ror that reflected your heart’s desire, and it showed you only the way to thwart Lord Voldemort, and not immortality or riches.
There are no more uses of "thwart" in the book.
Show samples from other sources
"I assume you did it to thwart me," Ty said.
Cassandra Clare -- City of Heavenly Fire
The time spent denying coverage, arguing, dismissing, thwarting—surely it was more trouble than simply granting her parents access to the right care.