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Unfortunately there is no way to stop a period once it starts .
To stop a period you have to start medication three days before the expected period is due to begin .
Check this link for future reference .
If you are going to have sexual intercourse, try to get your partner to straddle you while you are lying on your back: this will help to slow the bleeding. Lay down some spare towels or sheets beforehand, so that it will absorb any excess blood. You may want to think about wearing a diaphragm or cervical cap during sexual intercourse. These will protect you from pregnancy and work to contain any menstrual fluid. However, these barrier methods of birth control should not be used on an everyday basis to collect menstrual blood. Although they are similar, diaphragms and cervical caps are not the same as menstrual cups.
Source - http://www.epigee.org/menstruation/sex.html