How hot does water have to be to kill sperm?

Not sure where you are going with this, any water (cold, warm or hot) will wash away semen and sperm from clothing, sheets or skin.

If someone has given you ideas about using hot water in your vagina as a means of Birth Control, don't listen. This is not correct and douching can actually flush sperm up into your uterus, which could increase the chance of pregnancy. When a man ejaculates during intercourse the sperm is shot up towards the cervix (opening to the uterus). Even if you were to try to wash it away immediately, it is too late the sperm would already be in the uterus, out of reach.

The only way to prevent pregnancy (while sexually active) is to choose a reliable method of birth control and use it as directed - condoms with spermicide, a diaphragm, a cervical cap, Today Sponge, is 85% to 98% effective. Hormonal birth control taken/used daily or changed weekly (the Pill, the patch, NuvaRing etc) is 90% to 99% effective. An IUD, Implanon, Depo-Provera is >99% effective.

Emergency Contraception can be used to prevent pregnancy after having unprotected sex. Plan B or Next Choice is available without a prescription to anyone 17 years or older (under 17 it is available with a prescription). It is not as effective as birth control, but can be used any time you need a second chance to prevent pregnancy for up to 120 hours after unprotected sex. It is 95% effective if used in the first 24 hours, 88% effective between 25 and 72 hours and somewhat effective from 73 to 120 hours.