how do i cancel my idea membership
You could just allow it to expire. If you want to get some type of refund, you will have to contact IDEA. There is a contact phone number at the top of every page.
Are you just not finding it to be useful? You may want to look into how to get more out of your membership first. I find my membership to very useful, but it took me a while to find out all of the different things available to me and how to access them.