How do you block a sender in imessage Monterey?

Use Messages in iCloud

And on the iPhone Setting >>> General >> Messages and enable Text Forwarding.

And on the iPhone Setting >>> General >>> Messages and make sure the Apple Computer By Name is listed and Enabled ( Green Slider )

Since you have already marked the previous suggestion as Solved the remainder become mute.

Block messages from a specific person or number

1. In a Messages conversation, tap the name or number at the top of the conversation, then tap 
2. at the top right.
3. Tap the name or phone number to view the contact card.
4. Scroll down, then tap Block this Caller.

• Open Messages
• From the menu bar click on help
• Type block into the dialog box
• You'll see the answer listed
• This tells you what to do in the Messages Preferences

Block phone numbers, contacts, and emails on your iPhone, iPad, or iPod touch - Apple Support

On Iphone -> Settings -> Messages -> scroll down to Blocked Contacts

I would expect that the phone and mac will synchronise settings

OK