How to delete my created but closed giveaways ?

You can't, but you can ask support to give you an additional giveaway slot if you have none left.

Have you given the winners of your closed giveaways their gifts yet? Once they mark as received you should get open slots again.

*Edit - Oh. I see that your other giveaways were removed and not just closed. You will either have to wait until they are free again or contact staff and they may give you more. Your problem is because you cancelled your other giveaways. That's why it asks several times if you are sure before you click the button to submit. Read the F.A.Q. if you are having problems figuring out how the site works.

Explain to support what happened, wait a few weeks (maybe months) for a reply.

Thanks a lot to you all :) i will see what can i doo :)