I bought a bracelet from buzzbargain (Sorry for advertisement)
Not much of an advertisement given the rest of your post :)
then they send me an email saying that we will deduct 49.9 USD after a month for a subscription to which I didn't subscribe.
They state it within their Terms, specifically 8.1 - 8.3. Effectively, you weren't just purchasing a bracelet; you were subscribing to their site which permits you to purchase the bracelet at their low price. And by default, it looks like you had their package b:
(b) 12-month membership: - no initial charge; then £49.95 on the 45 th
day after the day you purchase your subscription; and then £49.95
every 365 days.
This is not an uncommon scam, er, "business model."
So I want to ask will they deduct me that 49.9 USD without my permission?
Very likely, unless you cancel. (Not completely unlikely, even if you do - so pay attention!)
If yes, what am I supposed to do now?
Those same terms indicate the procedure for cancellation:
8.5 Your right to cancel your subscription. You can cancel your subscription at any time by calling [1-855-984-9495] or emailing us at
[email@example.com]. If you contact us to cancel, we will
terminate your subscription within  working days and your access to
our VIP store will terminated. We will also discontinue any future
charges to your payment card for any future product shipments but
please note that you will still need to pay for any products that have
already been shipped prior to cancellation. If a billing period has
already started, you will be charged pro rata for the relevant period
up to the date of cancellation.
I recommend calling and emailing them, and keeping records of both - time of call and name ("Bob") of whoever you speak to, and your email and any responses. Make it clear that you are cancelling, that you were not aware this was a subscription, and that you will initiate a chargeback dispute if the annual fee is charged despite your attempt to cancel.
Print that email out. Wait. Watch your statement.
If they do charge you, take the printout to your bank and file a chargeback.. I have had to do this in your exact situation, and I got my money back.
And if your cancellation email/phone is calm and clear, it's likely the merchant won't even try - they lose money on chargebacks, and it's not worth their time to charge someone where they know they'll likely be contested and lose.
I don't know of a way to block a transaction before it happens - and I asked last year, when someone managed to start charging their Netflix to my card. The bank said all they could do was chargeback the charges once they cleared. Netflix was able to chargeback them from their end, which saved me the paperwork.