Plans: how to leave "Pro" plan and stay in "Base" -free-plan?

Hello, first of all sorry for my English (I write from Italy)!

I have been paying Pro Plan until June. From July on, I've not been interested in Pro Plan any more: it is enough Base Plan for me.

My problem is that I left Pro Plan just quitting to pay.... and I keep receiving invitation to pay... I don't know how to stop receiving them, and how to "communicate" with the system my desire to stay in Base Plan.

I attach here a screenshot of my billing status.

Thank you very much in advance for any help!!

F