You can use call labels (under ACD). First set up a label (blacklist / spam / junk) and then simply add the numbers to be blacklisted to the phonebook, assigned them with the label and a value (e.g. 1) then using action conditions under ACD you can control how the call should be handled depending on whether the number has the label assigned or not.
Don’t forget to configure the action condition for both if the label is present and if not, plus remember the hangup as otherwise you will end up with a dialplan loop.
One more thing. When testing use a prompt such as tt-monkeys or tt-somethingwentwrong before hanging up. Blacklist your mobile number and test with your mobile and another number to make sure the system is working as planned. Once sure, remove the prompts from the action conditions and leave just the hangup.
Hope this helps.