im using "User Frontend Pro" plugin
the "post tags" field has an standard value wich i set in backend
the U.F.P plugin has and option to hide fields in frontend
there is an field for the U.F.P. form wich is called HIDDEN FIELD
you can add "metakey & metavalue" of an other field in that HIDDEN FIELD options
i think it will then hide this specific field in frontend / im not sure
i dont find thecorrect "metakey & metavalue" for the "post tags" field wich should be hidden in frontend.
other solution /hide the field with custom css