1.        Set options so that no custom fields will be shown in “blank” (no) setting in Item dialog.  See figure 1 below.

2.       Make an Item Setting that includes your restricted custom field(s) and for this item setting, set “User Group” to “sysadmin”.  This will make sure that only members of the sysadmin group can access the Item Setting that gives access to the restricted custom field(s).

3.       In all other Item Settings, make sure your restricted fields are moved to the “Hidden” group category in the Item Setting.

4.       Remove access to Item Setting in the permission system for other groups than sysadmin group

Figure 1:

Figure 2: