It is a straight red under rule 12 if Roberts had been thwarted, but the ref IMHO did the righ things and let play continue. He could have yellow carded him for unsporting behaviour or a 'foul' but what did it matter.
A player is sent off and shown the red card if he commits any of the following seven offences:
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denies the opposing team a goal or an obvious goal-scoring opportunity by deliberately handling the ball (this does not apply to a goalkeeper within his own penalty area)
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