You can cut that out if you're able to do without an answering device. No kind of answer makes it impossible for callers to leave bogus threats, possibly stopping them from wasting time calling you. If they are able to get any answer, the fake IRS callers will leave you messages. You can stop them as unanswered numbers are worthless to them.
They've got no chance for pay if nobody plays. Without any kind of answer, these crooks are out of luck. Let the phone ring and cut back on worthless calls.
You can stop them: Do not answer unknown calls and do not call them back. Consider getting a call blocking device (or app for your cell phone)-they're available online and start around $45. http://800notes.com/nb/search?q=call+blockers