Quote:
[G23]
Robots on the same Alliance may not work together to blockade the Court in an attempt to stop the flow of the Match. This rule has no effect on individual Robot-to-Robot defense.
Violation: Technical-Foul
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Emphasis mine, but it stands to reason that holding all/some of the balls in the alley would stop the flow of the match.
Even if you did hold the balls, that could be considered controlling, so lets say that you're holding only three balls, if they gave you the technical foul mentioned above, those balls will only make up for the penalty (if you got them all in).
tl;dr: Rules are too vague, not worth it.