Exercise can not stop you period however it may delay it. Recently in order to lose weight and get back in shape I have been walking 10,000 steps a day about 5 miles and my period is about 2 weeks late. Due to a change in diet and exercise my period is late. This does not mean thought that you should exercise to avoid your period, because messing with your period could bring about harmful changes to your body. As much as you might want to avoid it , its better to have it and get it over with. Hope this helps =)