How To Save My Garden From Outsider Cats?

Hi everyone, I am a newbie on this forum and I need some help friends.

I have three cats and I have no problem with them. But the problem is with the outsider cats. As I have a vast garden and my husband loves this more than me. But recently we are noticing some outsider cats spoiling plants and saplings. Sometimes they use the garden as a toilet and latrine. We dont understand what should do and how to stop them to come our garden. Could anyone please suggest what to do? Any kind of suggestions will be highly appreciated.