Usually if I throw the ball over my neighbour's garden I would kindly knock on their door and ask them whether or not I can have my ball back. But, I don't think I have ever been in a situation where as a neighbour would actually not return it. Anyway, I won't really call it 'stealing' and I don't think it is actually 'theirs'. You paid for that ball and you had mistakenly threw it over to their garden. So obviously the ball belongs to YOU and only YOU. Not the neighbours.
