using our property

Washington state near Sumas
My inlaws have a mobile home here. They have not been at the property for four years. We all live in BC. My husband and I went down to check out the property and discovered that a neighbour is storing vehicles, including a mobile home and other junk on their property. He also has stuff in the mobile home and has been using it as a workshop. My inlaws would like to sell this property, but it is obvious that we will have to do a lot of cleanup first. Is there anything we can do to syop this neighbour from using this property that does not belong to him

Yes---just tell him no more. That is your right. You may want to give him a neighborly bit of time to move the items.

You can call the police for trespassing if need be.