Do not warm food that has been touched by a customer. No exception.

How are they stealing food?

Call corporate as a concerned customer to complain about the homeless in the lobby. The DM will get the email and allow you to kick out non-customers. It's sorta sad but Starbucks isn't a homeless shelter and it's truly not fair to Partners or actual Customers.

I work in a very nice down town, I can 100% relate to this. The homeless people we know are either addicted to drugs, always have fresh needle marks, smell bad, are violent, or way too loud. My store is close my High School I attended before graduating, I've been followed to work, not once, not twice, but three times. Recently I had to call the police because there a new homeless person that I've never met before who was smoking a cig while sitting on our outdoor furniture. He was harassing women, especially young women and minor girls. He refused to leave and the police forced him to leave the area, we had him identified and all employees working filed a formal restraining order against him in addition to being trespassed.
The city I live in also has amazing homeless programs, local shelters allow people to come in go throughout the day and during the late night if they're returning or going to work, local churches and technical colleges invest a lot of time and money to get people their GED's, and help people apply for certain benefits or assistance.
Out of the dozens of homeless people that come in while I'm working, or throughout the day, there is only one that is actually getting assistance and is not a nuisance to our customers, or the store partners.

I mean who the fuck cares about homeless people stealing just so they can have something to eat/drink. You realize Starbucks is still making billions right and can’t even pay us living wages? Just give them the coffee refills and leave them alone.

As someone who was homeless as a child, you are speaking from an extreme place of privilege. If they are being violent towards you or other customers, I understand being uncomfortable. If they are actively doing drugs or getting drunk in the store, yes they need to leave. But they are human beings going into a public space that we consider to be “the third place” for maybe air conditioning in the brutal heat, somewhere to seat without being arrested for loitering, sometimes utilizing the WiFi to fill out job applications. Are they causing you harm by being in your store? Is it coming out of your pay or anyone close to your level to give them the smallest gesture of a free beverage? I’ve had college students and construction workers and business people sit in my lobby for hours and hours and nobody complained because they weren’t bothering anyone. If the fact that they’re homeless (which I’m curious to know how you KNOW they’re homeless) is the only reason you don’t want them there, it is discriminatory and shows you view homeless people as subhuman. People like you are why my mother, siblings, and I would have to spend hours outside in the cold and snow waiting until we could go back to the shelter we were only allowed to sleep at. Let them sit.