What is the ruling on one who intentionally goes to the Friday prayer and enters the mosque after the sermon has ended, believing that the sermon is useless because it contradicts his madhhab?

It is not permissible to be absent from the Friday sermon except for a legitimate excuse. The Imam's divergence from the adopted madhhab (school of thought) is not a legitimate excuse that permits absence from it. Whoever is absent for that reason is sinful.