In terms of legal obligations, sure - however, for SaaS, it's almost never worth it. If it's "we charge you at the start of the period", then you received no goods, they received no payment, there is no legal obligation. If it's "we provide the service and charge you at the end of the period", then there is a legal obligation, but the cost to them to collect is probably too large; they could always send it to a debt collector, but good luck proving that debt ("the issuer of the debt provided access to a service" "...that I was unaware of and never used? Sounds fraudulent").
In terms of merchant choosing to do business with you in the future, they may or may not have a choice; depends what they use to identify you with. Certainly, if they don't make canceling easy, it's probably not the kind of business you want to deal with.
All that said, this is why anything that auto-renews, that I don't know if I want to keep renewing (i.e., will I still be using it at the end of the trial period, end of the month, end of the year), I immediately cancel. If it's a trial and that terminates access, I will take that as a sign not to use them. If it's paid and that terminates access, I will also take that as a sign not to use them, but I'll also email them and basically say "hey; I paid for X period, wish to use it for X period, but am unable to use the service for X period. I either need you to reinstate my account, sans auto-renewal, or I expect a refund". That tends to get a response, since otherwise -they- are legally on the hook.