How to permanently block spam emails on iPad?

Spammers spoof the sending address to make blocking ineffective, so the same email can come from a different address. Suspected spam emails will go to your Spam folder for further review as it is possible a suspected email is actually legitimate and you need a place to find it, while a previously blocked contact will go to the Trash folder. There is no perfect method. I found that out when using a Spam filter with my cellular provider that blocked spam calls and unfortunately found that it also blocked calls from the SSA and my Health provider and I never received those calls and had to turn off the spam blocker.

The Mail Filter option in iOS and third party email blockers may be helpful, but again not perfect. Prevention is the best way by controlling who you give your email address to. Apple does have a Hide My Email service included with an iCloud subscription that allows you to create alias email addresses that you can use for different services where an email address is required. That way when you see the spam emails being targeted to a specific email address you can turn it off where that email address is no longer used and you will not receive any emails sent to it. You are then able to identify the service that sold your email address. Unfortunately this will have little impact after your primary email address has already been exposed to the spammers and works best when using a new primary email address.

Create and manage Hide My Email addresses in Settings on iPhone - Apple Support

If you ever find a way, you would be rich.

Unfortunately spam cannot be permanently blocked, sorry.

It's an issue everybody contests with daily.

Addresses can be spoofed, changed, modified so blocking one, would not be effective against a different one sending the same email.

Also, as mentioned, aggressive filtering can block things you do want to receive, it's a very difficult balance to attain.