in thunderbird why is there no block only junk as emails keep reapearing instead of being blocked

If it were that simple to block junk/spam mail we wouldn't have a spam problem at all.

The first line of defense should be your email providers spam filter, assuming your email provider provides it. If your account is IMAP, spam messages will be moved to your accounts Junk/Spam folder. If your account is POP, you'd have to check your Junk/Spam folder on the server via webmail, as it isn't visible in Thunderbird.

The second line of defense would be the Thunderbird built-in junk mail controls. See https://support.mozilla.org/kb/thunderbird-and-junk-spam-messages for more detail.

So after all, spam messages get filtered to your Junk/Spam folder, so that they aren't visible in Inbox. But it isn't possible to just block them.

Note, it is recommended to check your Junk/Spam folder on a regular basis for false positives.

Some e-mail service providers allow users to write scripts, e.g., with Sieve, to reject or discard messages that meet users' criteria. If users reject messages, they are blocked, i.e., they never appear in any folder.

Some e-mail service providers have settings for their spam/junk filter that allow users to set more aggressive filtering so that more messages are trapped as spam/junk.

Remember that one must train Thunderbird's filter.