Situations like this is why I am not to crazy about using food thrown on the ground to train dogs...not related to your issue...just "thinking out loud."
If you feel for certain that the grape eating can be a life or death situation??? And keeping her away form a specific area is a non viable option???
Then training in "grape" avoidance is the solution! Never thought I would be typing that???
As I see it there are going to be two ways to achieve this and either way is going to be "uncomfortable" for your dog!
Your going to need tools to do this Jeff Gellman talks often about bonkers. You take a small towel wrap it up with tape set the "bait," in this case, it would be a handful of grapes you gather up and place them where the dog can get to them.
Since you don't want to encourage counter surfing, most likely place them on the ground. And "perhaps' place an acceptable choice a couple of feet away.
If the dog makes the proper choice she gets praise and the treat! Maybe a different treat instead of the one in the bowl or maybe the one in the bowl is OK?? Don't know but the right choice is not as important as the "wrong choice!"
If the dog goes for the grapes, you say no and throw the "bonker" at her! IT's a towel it won't hurt the dog but it will have an impact (no pun intended) you want to scare the crap out of her! Your trying to teach her that grapes are bad news!!!
Alternatively the actually mechanics my be tricky but a bucket of water thrown on here will have more of an "impact!" I used the water thing with my car chasing, Basenji as a kid. Set the dog up dosed him a bucket of water! Only took once! If it worked with a Basenji, it will certainly work with a Boxer!
Those are the essential elements of what your looking to do, if you do it manually as it were. I have no idea how long it would take for the grape avoidance to stick?? Mostly likely if you can manage the bucket of water bit...it would only take once.
And I would think you would want her to be almost in the process of eating the grapes to drop the hammer...not just looking or sniffing the air??
Now if you want a more sophisticated route?? The E collar is your tool of choice! Done properly "grape avoidance" can be taught in a few hours!
A trainer would put an e collar on your dog,set the bait and when the dog goes for it...light them up like a Christmas tree! Well maybe a small nick depends on the dog but I like being dramatic
It's in essence the same process they use to teach Rattle snake avoidance classes! They put an E collar on the dog and use a caged defanged rattle snake! Guaranteed results, you can't have hunting dogs search and rescue dogs screwing around with rattle snakes!
And out here family pets and there owners are equally at risk!
Further info on E collars Crittering is the essence of it:
Game Chasing (Crittering)
I just grabbed the first "Crittering" link I saw.
Lou Castle Crittering And Aggression - German Shepherd Dog Forums
The E collar is not my thing, the way I explained it is more of a hands on approach. But Lou Castle is the expert! If you register on the board you can:
1) Start a thread or
2) Contact Lou via PM!
He is a member there...ironically a lot of Pros are!!
So yep there are things you can do, including "ignoring the behavior" and "hope" it works out??
I am not a big fan of either of those approaches myself!