You can mark it as 'internal' though. It's a setting for a field on the 'Code generation info' tab of the entity editor.
By default a field is 'public' (which is driven by the default setting in the project settings -> Entity Model -> LLBLGen Pro Runtime Framework). You can mark the field as 'internal' by selecting the field on teh code generation info tab and then setting the 'Field property is public' setting to false.
This gets tedious for a lot of fields, so you can do that in one go for multiple fields/elements by using the 'bulk element setting manipulator' dialog, using Tools-> Bulk element setting manipulator.
then select as element type at the top right 'normal field' and scroll the grid so you'll see the last column, 'Is foreign key'. Sort that column so you have all FK fields together, select all these rows in the grid and change the setting in the property grid below it.
Click OK.