Is your requirement to prevent them from re-using their last password, or to prevent them from ever repeating the same password again?
The best way to prevent password re-use in b2c is this custom policy for scenarios where you need to implement a password reset/change flow where the user cannot use their currently set password:
https://github.com/azure-ad-b2c/samples/tree/master/policies/password-reset-not-last-password
As of now we do not support enforcing password history in B2C. You can create a banned passwords list but there isn't an out-of-the-box recommended way to do what you are asking. We recommend instead using Azure MFA to secure the accounts. https://feedback.azure.com/forums/169401-azure-active-directory/suggestions/34839976-aadb2c-password-history-policy