That is SO annoying!!!  Now maybe I'm just "jumping to conclusions" but it doesn't sound like these people might be the MOST upstanding type neighbors...especially given the sister was KICKED OUT (or 'put away') for selling DRUGS in the house!!!  
But nevermind THAT....the MAJOR ISSUE with me is the woman INITIALLY asked to plant things in the OPs yard/garden!  (WHAT THE HECK?!?!?)  Do they not have a yard themselves?  I mean COME ON!  Anyways...the OP told her NO!!!  NO NO NO NO NO!  No means NO.  PERIOD.  This DINGBAT apparently is DEAF and DUMB that she 1) didn't hear the "no" to the whole gardening in the OP's yard and 2) felt that taking fruit from the 'off limits' property was JUST FINE!!!  
In lieu of the whole bacterium thing...you COULD send out a note to 'all' the neighbors about how you and your hubby had used rather TOXIC sprays to try to control the diseased blueberries in your yard both last fall and in the spring.  You could ALSO let "them" know you contracted LYME disease while gardening on the property!!!  (This would all be in response to the potential pickings anyone may've noticed in your yard this year.  You know...ooooh...look at all the blueberries....hmmm...they're still there....let's go pick some hon.....)
Maybe you could get your neighbor to take PHOTOS of the violators IN THE ACT?!  (Before he chases them off....with whatever long-barreled method he sees fit).  Either way...I WOULD file a complaint with the local PD if you can and possibly charge them for the amount of fruit they stole.  I would definitely NIP THIS IN THE BUD ASAP.  LIke you said...these people are USERS.  They'll squeeze you for whatever they can.  
I have neighbors whom I GIVE veggies/fruit from my garden TO.  Then one of the neighbor's kid is always it TELLING me to give him stuff so he can sell it on 
ebay.  He (& his complainy pants of a mother) don't get JACK from me.  AT ALL.  PERIOD.  NO EXCEPTIONS.  She'll even ASK me how my veggies are coming along....I tell her 'fine' and keep going.  She wants to complain about me while she sits at home all day drinking....she can garden HERSELF!  
Sorry...rambling.  Let us know how this works out and I hope you're feeling better.  Lyme disease really sucks and it becoming a lot worse lately.  Take care!