How to stop from-spoofing?

Unless you can close down the people who are sending the emails then you won't be able to, other people are likely to have received the same emails purporting to have been sent from their own accounts. Spoofing the sender's email address is relatively straight forward, it doesn't mean that you sent the emails.

Don't click on any links in the emails (including 'unsubscribe' links, that just confirms that the email account is active and is likely to result in an increase in emails), and depending upon what your email provider supports you could log into your account on your provider's website and flag them as spam and/or set up rules/filters based on their content so that they get sent straight to trash.

King-Penguin gave you some great recommendations above. The fact that you are seeing emails "sent by you" does not mean your own account or computer has been hacked. I could be hacked but it does not necessarily mean that, in fact in nearly all cases it would not be your computer. What happens is that someone who has your email address in their address book, anyone of your many friends, acquaintances, business contacts, etc. got hacked. This could be any of hundreds or more computers who have you as a contact. Then the hackers have access to all those addresses and spoof them to their malicious intent. About all you can do to stop this is follow King-Penguin's suggestions.

Your iPad does not have a virus or trojan unless it was jailbroken and I doubt you did that. Just to be on the safe side on your computer, I would do clean restart. Assuming you have a PC, not a Mac, I would do virus scan with your antivirus software (make sure it is up to date) and then use a malware program like Malwarebytes to further check. On a Mac there is not much to do, I trust the security software built in on my rMBP and I have never had any problems. On my Windows PC, that is another matter, nothing serious but Malwarebytes has caught a few things. Hope this helps you...

Change your email password. It sounds like you have been hacked. -AJ