Pressure to eat junk food at work

Ok so I try to eat clean for the most part but when I get to work it's a different story.

People bring food all the time, and as much as I try to politely say no thank you, they keep pushing me to eat constantly. It's mostly unhealthy foods and some co-workers will bring food for each person working on that day, so then I feel like I have to eat it so I don't offend. Even though I tell them I brought my own lunch they still insist and I know it comes from a good place but still. Any advice on how to deal with this?