Unable to Unsubscribe from a newsletter

I hope someone can help, I have recently had to drop my internet from 25g to 5g due to rising cost. That being the case I have Unsubscribed many of my newsletters without problem. However, there seems to be one company that will not let me do so, they provide the "unsubscribe" clickable top and bottom of the newsletter but it is obviously not working. I have sent 5 (nice, please unsubscribe emails) to no avail- 2 weeks ago. The company name, RETREVO, do not subscribe or you will receive DAILY newsletters you really don't want. Is there any other way I can unsubscribe? This has really got me hot under the collar now, I thought it was illegal not to let customers opt-out?
Hoping to hear from someone with some ideas please.
Regards Curley54