apple id sign in showing up on my iPad. how do I disable on my iMac?

Verification codes and passwords are Apple's strategy to assure your identity is valid and safe. They can't be disabled.

Very important document to read for two-step verification for Apple ID > Frequently asked questions about two-step verification for Apple ID - Apple Support

You can turn if off.

How do I turn off two-step verification?

1. Sign in to your Apple ID account page.
2. In the Security section, Click Edit.
3. Click Turn Off Two-Step Verification.
4. Create new security questions and verify your date of birth.

You'll get an email confirming that two-step verification for your Apple ID is off.