I did that some years ago, and I called them on the phone to do it. I cursed that MF up one side and down the other before I hung up as well. I actually think the guy was a bit understanding. Maybe he was not too happy with the RINOs either.
I had the opportunity when they called me for a contribution. Realizing the person on the other end was just a worker trying to earn a paycheck, I was polite but to the point. I asked him how old he was and he told me 20. I then told him in a calm manner that I had been a registered Republican for longer than he had been alive and that I was both saddened and furious to see the once Grand Old Party turn so far left. I told him I could no longer in good conscience support financially or with my vote a party that had abandoned me and my core values. He sounded as though he understood and told me that I was not the only person he had heard that from. That was 7 years ago and the party has not changed course, it continues to go down the sewer. I believe if more people would do what I did it would matter. They're not about to change as long as suckers continue throwing money their way. I haven't received another call from the GOP since.