Is there a way to tighten my belly without using my neck? 

Yes there is, you can perform the prone is ab hold, which will not require you to flex your neck at all. Lie flat on the ground with your elbows and forearms on the floor; lift yourself until your body is parallel with the floor spine straight, head looking down at the floor.  Hold for 10 seconds then relax, perform three sets of 12-15 repetitions.  This exercise will help to strengthen and flatten your midsection but will not require you to use your neck.  

I hear all the time that people want to strengthen their abs without using their neck. If that is you, have I got the exercise?    

While on your back bring your right knee up in line with your hip. Take your hands and press them against your right upper leg for 30 seconds. The entire time pulling your stomach in like you are wearing a tight pair of jeans. Keep your face relaxed. Switch and do the other side. For more advanced to both legs simultaneously.