The rule should work providing you have the correct address in the rule. Also, no rule above it that would stop that one from acting on the message.
Rules only apply to new messages coming into the inbox. If this is an imap account and the message is downloaded and read on another device or website first, it will be shown as read and the rule won't work when downloaded to the computer where the rule is set.
So, if you only have one computer, check the rule and see that the address of the sender is correct. Place the rule at the top of the rule list.
A better way is to go to the website of your mail provider. Most have settings to block specific senders at the server.