Carefully and thoroughly read this support article about how to delete your Apple Account—> https://support.apple.com/102559 You have to be able to sign into the account to request deletion.
You will apparently now have the additional hurdle of the—what I presume are forgotten—security questions. Several years ago Apple transitioned to using two factor authentication instead of security questions. You can continue to use security questions if you already use them and they are working for you, but if you have problems then they no longer get direct support. You can try the directions in this support article https://support.apple.com/HT201485 but the article is labeled "archived" and may contain information that no longer applies. If the instructions work then fine; if they do not (e.g., you see “not enough information”*) then you will have no option but to change to the newer security method, if you can. Over the past few years Apple has been persistently encouraging users to switch to the newer two factor authentication security system. For those who have disregarded their encouraging, Apple will likely only help you to now change to two factor authentication, if even that is possible. Follow the instructions in the "Turn on two-factor authentication for your Apple ID" section in this support article --> https://support.apple.com/HT204915 to try to change to two factor authentication.
If you still need help, contact Apple Account support at this link ➞ https://getsupport.apple.com/?caller=cups&PGF=PGF63005
Select "Other Apple Account Topics", then "Security concerns or account compromised." This will give you a chat or telephone call option.
If Apple cannot, or will not, help you transition to two factor authentication then it is quite possible you will have to simply abandon the old account and everything it contained. In that case, if you were using the Find My service, the Activation Lock support request support article at: - https://al-support.apple.com/#/additional-support may at least let you regain use of your equipment. If not, you will also have to say goodbye to the device as well.
Good luck!
*More about identity verification problems here —> https://discussions.apple.com/thread/256247991?sortBy=oldest_first
What makes you think the account is being "hacked". As said in another post, these could be fake messages. Even if they are real, you will have to convince Apple you are the true owner of the account before they will deal with it. Read this document if you think your Apple Account has been compromised. --> https://support.apple.com/HT204145
Use the information in this document to check your Apple ID device list to find where you're signed in --> https://support.apple.com/HT205064
Related materials:
Personal Safety User Guide
https://support.apple.com/guide/personal-safety/welcome/web
> open the Table of Contents and review the articles
A document with general information about security and your Apple ID --> https://support.apple.com/HT201303
Contact Apple for help with Apple ID account security. This page provides country-specific Apple Support contact information ➞ https://support.apple.com/HT201232