There are two places that you can have default values: in the table and in the form/report. I don't know which one you checked, but I'll give instructions for both. For the table, open the table in design view and then click on the field that is being populated automatically. In the field properties on the bottom of your screen, there is a line for default value. For the form/report, open the object in design view, click on the control that is connected to the field that is being populated and view the properties for that control. In the data tab, there is a line for default value.
You might also check the form's OnOpen, OnLoad, OnResize, OnActivate and OnCurrent events. Each of these events occur (in that order) when a form is opened. It is possible that in one of these events there is some code setting a value to the controls.