Can i stop a panorama manually on my Nikon D3300? I have looked in my user's manual and it just says it will stop automatically at the end. Is there any way i can stop it before?
Not really. This is typical of such feature on most cameras. Some however detect when there is such a jump that it cannot continue stitching, so you can try jarring the camera at a perpendicular angle to the motion you were previously following.
There is not much else to do if it does not stop. Powering off the camera will result in the panorama not being saved at all which OK to do if you decided that it was not good anyway. If what you want is just a shorter panorama, it is easy enough to crop in any imaging software, including free ones.