bulk block the IP addresses in exchange online?

Looks to me like you want to block auth attempts, not messages coming from those IPs, right? If so, the best way to do it is via Client Access Rules, which allow you to specify allowed/blocked IPs per protocol: https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/client-access-rules/client-access-rules

Or you can even block legacy auth altogether via auth polices: https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/disable-basic-authentication-in-exchange-online?redirectSourcePath=%252fen-us%252farticle%252fdisable-basic-authentication-in-exchange-online-bba2059a-7242-41d0-bb3f-baaf7ec1abd7