Same here, bought two new pillows from spotlight a year ago and have been bombarded by their ads since. Rang the unsubscribe number, it's always disconnected.

Once you purchase an item online or in person at times you will be asked for your Email & or Phone No. this is regarded as an automatic acceptance to receive notifications from the Store/s concerned.
Either way you will get Junk, but by Australian Law you must be able to "unsubscribe" this is found at the bottom of the notification and a simple click should "unsubscribe" you. Some will make you respond with text I have found these are usually based in other Countries.
But the law still requires them to act. Not supplying a email address or phone number usually won't help your purchase, I would suggest a secondary email address for all such transactions.