People come to you with their problems because you must be a good listener and give good advice. That's what they get out of their conversations with you.
At the same time, in every relationship, both parties get something out of it. So you need to look inward, and figure out what you get out of being the "go-to person". At the end of your question, you remarked about your concern regarding "diminishing your value". That suggests to me this role makes you feel valued and important to your co-workers.
If, in fact, that last point rings true, you need to find a way to get your need (to feel valued) another way. There is nothing wrong with your need to feel like you matter. The problem is, this particular way of getting your need met, is interfering with your job. So perhaps in the future you can get together with your co-workers after work, either in person or on the phone.
You sound like someone who is truly considerate of others. In response to how to tell people without them feeling hurt, tell them the truth. Let them know you are willing to be available to them, but at a time that is convenient for both of you. Chances are, most of their concerns and problems can be discussed at a later time. They will know you are sincere when you initiate setting up a specific time.
In addition, it might be helpful to take inventory of your other relationships and interests. There are many opportunities to make a difference in our community, where you will be an essential part of the lives of others. Some examples are Big Brothers/Big Sisters, Compeer, nursing homes, Jewish Family Service and Habitat for Humanity.
In the end, your co-workers will respect you for making sure you're concentrating on your job during the work day. And as a side benefit to them, waiting to tell you their story may result in them solving the dilemma themselves!