How do I stop my cat from guarding the litter box?

I have three cats. Two are female and have lived together for at least 15 years now. They are, of course, old. The third is probably almost four years old, and a male. All are fixed. The male cat gets along with the oldest of the two females, but fights incessantly with the other, following and antagonizing her. And now, she won't go into the litter box area all the time anymore. She still uses it when he isn't around it, but he likes to stay in the area and won't let her go. So she pees all over the house. I'm at my wits end, as I cannot afford to take her to the vet right now, and this only started about a month after the male cat came to live here. Any suggestions? Could it be physical, or could it be jealousy, and is there any way for me to stop it without getting rid of a cat?