This petition was rejected

The Government e-Petitions Team gave the following reason:

We can't accept petitions about appointments, which includes calling for Ministers, including the Prime Minister, to be sacked.

If you're concerned about a particular policy or decision that's the responsibility of Government or the House of Commons, you could start a petition about that.
Petitions calling for action on policy are more likely to be an effective way of achieving the change you want.