Blacklisting phone numbers can save you from persistent telemarketers of from anonymous robo-calls.
You can either blacklist by phone number or by caller ID.
In the 3CX Management Console, got to Settings > Blacklisted Numbers
Click "Add." 
There will be two fields to fill out: "Incoming caller ID to be blocked" and "description."
Under the first, enter the full number or caller ID that you want to block. Note that they MUST match precisely, including case: "jeremy" and "Jeremy" are considered different. (You can use an asterisk to cover both situations: "*eremy".)
Make sure you pay attention to what information your phone service provider displays for you, because the information needs to be exact. For example, some providers display the country prefix (in the US, the "1" before a 10-digit phone number), while others don't. If you provider does, you need to as well. The same precaution applies to blocking by caller ID: It must be exact or it won't work.
The description field is simply for you to know why you blocked the number
After you've filled the information in, click "OK" and you're good to go.