Disable Waiting for Activation Popup

I faced this problem before. You can try to use this trick:

1. Go to Settings ~> Message and Turn Off iMessage + Facetime.
2. Turn on the Airplane Mode.
3. Turn on Wi-Fi.
4. Turn on iMessage like the first step. The system will ask you for your Apple ID If it is not added yet.
5. Turn Off the Air Plane Mode.
6. iMessage should work properly now.

Source.

Take out the SIM card, this should fix it. If it's not activated, you shouldn't need it anyways ;) I've often used iPhones without an activated SIM card, and after taking these out, the pop ups stopped ;)

Just take out the SIM card.

I have an iPhone 5c and it got really annoying having the "Your iPhone is not activated" message every 10 minutes. I took out my SIM card with a needle (because the iPhone 5c is different than other phones).

I haven't seen the message since.