Depends on where you enabled it

Global Settings From the Menu Bar select Tools-Options-General-Return Receipts

Per Account Tools-Account Settings-Return Receipts

No menu bar? Press the alt key.

macka4462 said

I enabled requesting a read receipt for one email that I sent out. But now I can't disable it permanently. I can only disable it each time I send an email.

Chosen solution from Airmail does not apply on my Windows 7 PC. The global setting Tools-Options-General-Return Receipts does not exist

The only setting which I can find which applies is: Per Account Tools-Account Settings-Return Receipts, as stated

You are correct. I left out Advanced in the original answer.

Tools-Options-Advanced-General-Return Receipts