Rashi: It is Yisrael.

Radak: It is a refusing nation; they rebel against Kel and his Mitzvos.

Malbim: There is one who hears Toras Hashem, and also believes in Him, but even so, he deviates and switches Mitzvos due to his desires. Also refer to 30:9:2:2, 30:9:3:1.

Radak: They were Bnei Kel, and they denied Him, that he is not their father, for they do not heed Him. A son who does not heed his father, it is as if he denies, and says that he is not his father.

Malbim: Some deny and say that Hashem did not command. This is worse than deviating due to his desires.

Malbim: The very worst is one who does not want to hear Divrei Torah at all.