In 2005 we had a very secure grabber as seen
here.
We could hold a tetra like in the picture and push people's arms/tetras as they were trying to score and it made a rather good defenseive robot that year because.
On a few occasions we tipped the other alliance (team 330), but this wasn't on purpose and they were able to get up very easily.
Not once throughout two regionals and championships did we get called for a penalty; If this stays about the same, we can probably play defense with an arm/torroid if it is purely for intereferance and not for destruction.