my tomcat harassing my cat!!!

hi everyone, We have 6 cats. two boys and four girls. Since shifting to Singapore eight months back, one of my tomcat (3yrs old) has been harassing one of my cats (6 yrs old). He does not let her move around the house. She is so intimidated by him that when he comes too close to her, she ends up urinating and at times defecating wherever the ambush happens. I have tried talking to him. But to no avail. CAn someone pls tell how can we stop him from bossing her?