"How shall I curse whom God has not cursed?" (Numbers 23:8) — For the Holy One, blessed be He, does not desire to curse them. For on the day that they made the Calf they were deserving of a curse, and not only that, but He blessed them, as it is said: "Yea, and he shall be blessed" (Genesis 27:33). And this is the meaning of "whom God has not cursed." "And how shall I rage?" — For He did not rage on this day, for the Holy One, blessed be He, blessed them, as it is written: "For the LORD's portion is His people, Jacob the lot of His inheritance" (Deuteronomy 32:9).
Why Balaam Could Not Curse Israel After the Golden Calf
Midrash Aggadah, Numbers 23:8
מה אקוב לא קבה אל. שאין הקב"ה חפץ לקללם שביום שעשו את העגל היו ראויים לקללה, ולא עוד אלא ברכו, שנאמר גם ברוך יהיה (בראשית כז לג), וזהו שלא קבה אל: ומה אזעום. שלא זעם בזה היום, שהרי ברך הקב"ה אותם, דכתיב כי חלק ה' עמו יעקב וגו' (דברים לב ט):