Volunteers come in all shapes, sizes and attitudes. They range all the way from the ones who are offering you their services because they’re sure they can improve on what you’re doing, to the golden one who states on their volunteer application “Put me in wherever you need me. I just want to help”. This is your Great Volunteer!
Serving Our Homeless Friends with Love
Human nature being what it is, you will get many more of the former than the latter. My advice is to listen to them. 
Sometimes, they actually are right and have great ideas. Don’t be so arrogant and defensive that you miss the opportunity to improve. After all, you’re out there to help your homeless clients in the way that’s best for them. That means considering anything that might be helpful and feasible.
On the other hand, after careful consideration if what is being suggested is something you know won’t work, because its: too difficult to implement, beyond your budget, dangerous, outside of the scope of your vision/mission, etc., feel free to say so. You are the boss. You get to make the hard/unpopular decisions. It’s up to you to steer the organization in the direction you and your board of directors have decided on.
After your potential volunteer has submitted their application, be sure to follow up with a phone call, if you haven’t already spoken to them. You can discern a lot in a phone call. Attitudes can be gleaned out of the conversation in very short order. There are people who don’t understand what you’re trying to accomplish and don’t want to. They’ve already decided they know all about the homeless and what they need. Dissuade this person from becoming your volunteer as gently and politely as you can. They will never work out. Even if someone has worked with the homeless for years with another organization, they’ve never worked with your organization.
Hold onto the ones who want to learn about the homeless and how your organization serves them. This volunteer will likely have the desire to make your client’s lives better. This is the volunteer who is teachable and will stay with you. They may even be the person who replaces you someday when you’ve gotten too old and creaky to lift those heavy food boxes!