I did this offer through a different rewards site that was offering more than SR. It is very easy to cancel - just call the number and tell them you want to cancel and say no to any deals they offer. It was quick and fast. I think it is a good offer.
I did this as well - but REALLY watch your credit card bill. I cancelled mine, and they charged me $99.99 a month later - they had no record of my cancel.
When you cancel - make sure you write down the confirmation number they give you. I was able to have them reverse it, but it was a shock to see it on my credit card bill!