Exactly, just prove your point - you want to change your eating manners so she shouldn't try to prevent that from happening.
Provocatively throw them unopened away.
-> She's gonna be upset and ask you why you did it
-> You explain it to her again, she will actually listen
-> Then you may do the shopping for her as well

Just to add to this...
What she is doing is passive aggressive behaviour. I.e. not telling you that she doesn't want you to become fit/slim, but rather sabotaging you by putting things in your environment to cause you to fail. You throwing them out is also passive aggressive, because it doesn't involve confronting her.
If she does finally confront you about the junk food in the trash, you should also discuss the passive agressive behaviour. (And that you were essentially giving her a taste of her own medicine.) It isn't healthy interpersonal interaction.
If she doesn't confront you, and you are absolutely certain she is aware you are throwing things away (for example she's seen you do it, said nothing, and continues to buy the junk), confront her again. If that doesn't work, ask about getting family councilling. There is something wrong, and this is just a symptom.
Eric