My iphone was stolen a month ago how to block the iphone so that they cannot use it?

If you had a strong passcode and turned on Find My they will not be able to get into your iPhone. If you remotely erase it they will be able to reuse it as that will remove Activation Lock. Also follow this article: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Follow these steps:

https://support.apple.com/en-us/HT201472

But OMIT Step 7.

go to iCloud.com, sign into the Apple ID you use on that phone, choose find my.

Select the phone, and choose remote erase.

Not only does this completely wipe your data, it also prevents them from even setting it up as a new phone without your password.