I'm not. sure I understand the question. If you are already pregnant, you don't need to kill the sperm. They will die on their own.
Here are some... Within 30 minutes after ejaculation, semen liquefy and sperms set free to swim upstream to cerviix, uterus, and to fallopian's tube to find its designated target - a timely waiting egg. How to kill them after insemination is hard but douche may help to make vaginal inside not suitable for sperms to survive and also dilute perm concentration. But clinically, taking Plan-B pill within 3 days is good.