If the imported OBJ file contains any material groups, then these will be listed in alphabetical order under the Groups panel. One of more groups can then be selected in the list, and the polys that belong to those groups are marked.
Detach Marked Polys
All marked polys are detached, exactly as if you had moved the mouse pointer over them and typed the Enter hotkey.
Various methods can be used to select groups in the list:
LMB
Selects just that group, deselecting all others. Click'n'drag to select multiple groups.
Ctrl-LMB
Toggles that group, leaving all others as they are. Click'n'drag to toggle multiple groups.
Shift-LMB
Selects all groups between this and previous selection.
