On the iPhone, go to Settings
Tap Messages
Tap Text Message Forwarding
Turn off the iPad
On the iPad, go to Settings
Tap Messages
Turn off iMessage
On the iPad, go to Settings
Tap FaceTime
Turn off FaceTime
On the iPhone, go to Settings
Tap your Apple ID name
Tap iCloud
Turn off Messages if it is enabled
Sign out of Apple ID on the iPad if you do not want messages synced there
Remove the iPad from trusted devices in your Apple ID account if needed