How do I remove IMessage from my iPad messaging?

Interesting question. iMessage is the iOS/iPadOS Messaging App/platform.

If disabled, you’ll not be able to send or receive any messages at all. If this is your intent:

Settings > Messages > iMessage - set to OFF

Perhaps you are instead attempting to limit which addresses or contact numbers can be used for messaging?

Settings > Messages > Send & Receive >

• [You Can Receive Messages To and Reply From] - selectively enable (tick) from the list
• [Start New Conversations From] - enable (tick) one from the list