You cannot disable the soft-delete, but you can use can use Azure CLI (or Powershell) to permanently delete the Key Vault. az keyvault purge --subscription {SUBSCRIPTION ID} -n {VAULT NAME}
Soft delete is implemented as a one-way operation and cannot be changed back once enabled. (This is mentioned in the enableSoftDelete property definition.) If you can share more details on why disabling soft-delete would be useful for you, and the specifics of your scenario, I'd be interested in starting a discussion with the engineering team for you.
During the creation of the Key Vault, you can set the retention period interval to a value from 7 to 90 days. You can't change that retention period after soft delete has been turned on though.
If you share the details of your use case and I will gladly share this feedback with the engineering team and start a conversation around this.
Thanks!
Marilee