Un-sync iMac and iPad?

Where you may have an issue is if you have the same email accounts

on both devices and the email account is IMAP.

In IMAP mail servers, all mail resides on the server and no matter what

device you log into those mail accounts with, they will seem "synced".

You can elect to have mail stored locally as well, but they will still "sync".

Basically, every device is looking at the same "mailboxes" with IMAP.

It sounds like you were using iTunes to sync the two devices. I would strongly recommend using iCloud to so this, this automates the process and will make like much easier for you. If you need help setting up iCloud please refer to the link that has instructions for your devices.

https://support.apple.com/en-us/HT208681

If you need further assistance, if the Mac is less than 90 days old you also have complimentary AppleCare telephone support that you can use. If you purchased AppleCare, you can call them after the 90 days has expired for further help. You can learn about this in https://support.apple.com/complimentary

Below are instructions for syncing using iTunes. You can do the reverse to un-sync.

https://support.apple.com/en-us/HT201253

Again, if you need assistance it's a good idea to use the AppleCare phone support available to you.

You don't indicate how you are syncing now, so how can we instruct you?

Your welcome.