Yep, but as the other commentator pointed out, you need to take all the ether at once. So if you wanted to charge 0.1 ETH a month for 6 months, you would need to take 0.6 ETH up front into the contract, which kind of defeats the whole point of the subscription model.

The smart contract could allow the user to withdraw the unused ETH to cancel the subscription but the fact that it still requires the entire amount upfront makes it an extremely poor user experience.

Yes you can. You could create an smart contract where you lock X amount of eth, and set up how much the other party can withdraw each month. Then is up to the provider of the service to withdraw the amount each month, which seems to me less an issue since they want to get paid.