Hey Leandrohailey,
I believe that your agenda is linked up towards your mail. In other words if you delete something on your phone agenda, it might be that it gets put back onto your phone. This due that your phone is connected towards a online agenda. If you go to settings> email, contacts, agenda's> You see under accounts all your accounts, if you press on one you see if the agenda is sync with that account.
So the problem might be online, for example...if you got a gmail account and you delete something on your phone, it should delete the event on your agenda online as well. Maybe something is preventing this for you doing it?
Hope it helps!
Friendly Regards
WhiteWolf