If you are an individual web subscriber, you can close your account yourself if you don't plan to use SIMTICS again.

Log in to your account at www.simtics.com, click on "Hi, {yourname}" (at the top right of the screen), then select My Account. Scroll down to Order History & Billing and you will see a blue button to cancel your account:

When you click that button, your account will be immediately closed, any recurring charges will be stopped, and you won't be able to log in again.