Are you able to become a forum helper if you have posting tip cards?

It counts against you but is not a 'black mark' preventing you from being considered at all.

how can you not have any warning cards??

Or is it a permanent blotch on your reputation

Very much as above. Posting tips are at the end of the day posting TIPS... they aren't a criminal record or really strikes against your account. More advice to how to improve your posting and stay within the guidelines of the site. If you've got blue, yellow.. or red cards then it's a different story.

If you have loads... and they recent.. we will likely reject you.
If you have some.. but they are old your recent post show you've learnt and grown.. we wouldn't reject you based on that.
If you have a couple.. and they are recent it'll be on a case by case basis. You might have accidently bumped an old thread.. where.. you know it happens.

What do you mean by "recent"? If someone got their last posting tip card 1.5-2 years ago and since then has been more regularly posting will they be rejected? I'm confused