I agree with the others, be consistent but be respectful with them.
Most of the FSBOs are very confident that they will be able to sell the properties themselves; at least at the beginning; and they don't feel the need for Realtors, but they do appreciate good suggestions and the courtesy of respecting their wishes.
You want to show an interest in their house, so try to stop by at their open house, and talk to them about their property and be helpful, but don't be pushy. You want to add value.
Step back a little at the beginning and watch for the signs closely. Follow up with them afterwards to get a feel and stay in touch. Once they trust you and remember your input, they will most likely let you have a foot in when they eventually decide to go with a Realtor; which they most likely will. .
Also, don't be discouraged by the very definite 'NO, I won't go with Realtor'. I have found out that they do change their minds and if you take their words at face value, you will be sorry that you did not follow up.