This can only happen if the SPAMmers have somehow obtained your iCloud login credentials
- there are only 3 ways that can happen:
- someone close to you "eavesdropped" while you logged in and sold your credentials on the BlackWeb
- you fell for a Phishing scam - for example, you followed a path from a clever email and entered your credentials into a bogus website
- the iCloud credentials were stolen from Apple's server in a hack = VERY UNLIKELY
Unless you fell for #2, and entered MORE than just your credentials, changing your password should be plenty to stop this
http://www.cloudpro.co.uk/leadership/6310/spammers-hack-icloud-accounts-to-messa ge-victims-in-china
...
“In the end, defeating these scammers will take a community effort – not just from Apple, but also from telecom operators and consumers,” said Anderson.
“Operators can protect their customers from unusual spikes in international traffic, Apple can and do[es] recommend using strong passwords and Two-Factor-Authentication on [its] accounts, and consumers should learn about and own their own personal security, paying particular attention to login alerts from new devices.”
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It is IMPORTANT that you report to Apple AND your phone carrier - this forum is NOT reporting to Apple
Report iMessages that look like spam or junk - Apple Support
reporting to your carrier and Apple may require using techniques in this section of above link
If you don't have iOS 8.3 or later, you can send an email to imessage.spam@apple.com with this information:
To report SMS and MMS messages, contact your carrier.
I would CC any emails to reportphishing@apple.com as that also sends the email to Apple Cyber Security partner
Security Awareness Training | Email Phishing Security | PhishLabs
https://www.phishlabs.com/