Why doesn't my filter work on messages that end up in Spam?

Filter rules run on mail that is delivered normally. When the spam filter flags a message, it goes to Spam and none of your rules run on it, including a rule that deletes or destroys it. That's why a rule against a flood of spam can look as if it does nothing: the spam never gets to it.

Make a rule act on spam

Add the action Never mark as spam to a rule whose conditions match the messages. That rule rescues them from Spam, and then every rule that matches runs as usual. So a rule with Never mark as spam and Destroy permanently deletes matching messages, even the ones the spam filter would have flagged.

Use it with narrow conditions. Never mark as spam delivers anything the rule matches, so a broad rule lets real spam into your inbox.

What a rule can't rescue

Mail from a sender you blocked, or from a server your workspace admin has blocked, always goes to Spam, whatever your rules say. To undo a block, remove the sender under Settings → Mailboxes → your mailbox → Block/Allow.

Spam that looks like it comes from you

Spam with your own address in the From line is forged: anyone can type any address there. Don't add a rule that delivers mail from your own address, because that lets exactly this spam through. Leave it in Spam, or delete it with a rule as described above, keeping the conditions narrow.