Keep snacking

I've got some problems on snacking and hope someone can give some advice.
[Apologize if the text is messy, long and full of grammatical mistakes]

I'm 25 years old, 5'5 tall, and currently 126lb. I'm not losing weight, instead I'm trying to "tone my shape" via weight training. I eat about 1300-1500 calories from regular meals, and drink about 2L-3L water every day. The problem is I keep snacking between meals. It's not because I feel hungry, bored or stressed. I think it is because the food is there.

I sit next to a room, which has a color printer, a water dispenser, and food/snack inside. Food/snack is provided by my co-workers. These things are placed next to each other, so whenever I fill up my water bottle or collect printouts, I see food. I can stop myself once or twice from eating them, however I always fail on my third attempt. Since then I tend to eat several bites every time I enter the room. I eat even I'm not hungry or I've brought my own snacks. I also found myself weak at portion control.

I hate myself doing it because I believe snacking is one of the reason that keep adding fat under my skin/muscle and therefore can't get the "tone the body" things done. When I snack I could eat 1000-1500 calories, in additional to my regular calories.

Thanks for reading I would really appreciate it if you could share your tips with me.

P.S. I didn't entered all the food I ate into my diary lately, but when I do I often get a record of ~3000 calories eaten in one day.