You can cancel your Issuu Pro subscription anytime and continue on Issuu's free service. Or you can delete your account completely. You can upgrade and downgrade as often as you like, but the free trial can only be used once.
Please go to: http://issuu.com/user/account/pricing to cancel your Pro subscription.
To downgrade to Issuu's free service please change your plan, otherwise delete your account.
The downgrade will take effect for all your documents (including embeds/standalone) and we will no longer bill you when your subscription ends, so it depends on your subscription period.

Note: If you delete your account in the middle of a subscription it is not possible to get a refund of the remaining amount.