I want to cancel my subscription today

But it tells me it will happen on March 16th. I want to cancel before then -- how do I do that?

That's the end date of your last payment which have been charged on your Paypal account on February 15, 2018. You can keep it that way. Your account will be automatically set back to the free plan on that date, and of course, you won't be charged again since the subscription is already cancelled. 

Is there a way to prorate my subscription? I really don't want to have to pay for the whole month! 

Sure. I have requested the refund to our billing team. You should get a notification from Paypal once the refund is made. We will also do our best to notify you in this thread. 

Thanks!