Semonics offers a free 30-day trial on all pay accounts (Personal, Personal+, Basic, Basic+, Plus, Plus+, Pro).
If you cancel your paid account before you are billed 30 days after your original signup, you won't be charged.
Once you cancel you won't be charged again, but you are responsible for whatever charges have already been incurred for the current billing period. For example, if your billing cycle is on the 15th of every month, and you cancel on the 20th, you'll still have to pay for the current month, but you won't be charged again after that.
You can cancel your account at any time by logging into your PayPal account.