In short: yes, you can.
For any plan above one month, you will be billed upfront at a discounted monthly rate.
These plans are 100% cancellable and once done so, you will be able to use it to the end of the initial term without worrying about it renewing.
For example, if you purchased a 6 month plan on January 1st, and decide to cancel on January 25th, you'll still be able to use it until July 1st.