Can't find how to block emails from undesired entities

Unsubscribing may not be a good idea. Anyone who would send you a virus is unscrupulous to begin with and isn't likely to honor your request. Instead, you've confirmed that yours is a valuable valid address that they can keep using and can sell to others for profit. But too late now!

You should be sure to mark those messages as Junk (keystroke "J") which will help train Thunderbird's Bayesian spam filter to recognize spam. That's a long-term solution because it takes some time for it to learn. Here's an article all about the filter:

https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages

As for the short term, read the section titled "Create filters manually". If the spam is consistently from the exact same address you may be able to create a Message Filter that will automatically mark it is spam or delete it. This is generally less effective because spammers usually change addresses every time, but it can help in some situations.

See what is possible at the server:

• Increasing the aggressiveness of the server's spam filter?
• Training the server's spam filter to recognize messages such as these two?
• Blocking these senders?
• Writing a script to block or filter such messages at the server?

If any of these work, you won't have to deal with these messages in Thunderbird.