The short version: is there a way to keep our neighbors cats out of our
yard and away from our bird feeders?

The long version: We have always kept our cats indoors, but our next
door neighbor doesn't share that philosophy. We have a number of bird
and critter feeders in our front yard which her two cats have started
staking out. So far they've killed 3 or 4 bird and at least one
chipmunk. We've talked to her about it a couple of times and she's been
very nice and apologized, but the cats are still being allowed to roam
free outside.  So far we've tried:

- Three different "cat repellent" chemicals that we've spread on the
boundary between our yards.

- One of the ultrasonic repellers, which seems to have absolutely no
effect.

The automatic water sprayers with a motion detector are not an option
for us because of our climate, and from what I've read, cats figure them
out pretty quickly.

I love cats, and I realize that they're just doing what cats do, but
they have become a real problem.  Any suggestions?  TIA.

Ken