Many Biblical commentaries wonder about the statement that Hashem hardened Pharaoh’s heart so that he would not let the Bnei Israel leave the land. The problem is that this implies that Pharaoh’s free will was denied him. This negates our teaching that a person has the freedom to make his own choice about how he wants to act and is therefore responsible for the consequences of his action.
A unique answer to this dilemma was given by one of the leading scholars in the famous Valozin Yeshiva. He points out that Pharaoh’s free will was not taken from him but rather his ability to withstand suffering was strengthened. He compares it to a person whose body is weak and his resistance to pain is very low. Every infliction is extremely unbearable. If, on the other hand, he has a strong body and is more able to hold out against inflicted pain, he can endure more.
This is what Hashem did to Pharaoh. He did not refuse him the will power to act as he saw fit. He merely gave him greater ability to withstand suffering. Hence he refused to give in until all the plagues were visited upon him and his people.