Classic Planning Poker rules suggest using a timer to try and avoid these kinds of situations. Anyone can start a two minute timer, and at the end of the time the team must estimate. Limiting the amount of time gives a degree of pressure and focus to the discussions especially when there is disagreement. Also it limits otherwise lengthy discussions where in fact everyone now agrees on the estimate.
I the past I have tried using this technique with some teams that had a tendency to get bogged down but with limited success. I have tried using a mobile phone, but people found it too complicated to set up quickly and easily. It was also too "personal". A stopwatch did not achieve the right results either - it was simply ignored probably because it counts up an not down. And I have never found a decent egg timer that is easily and accurately settable to two minutes (same problem as the phone). Then I stumbled across a kids 2 minute sand timer....
(Yes, it really is that pink!)
What a difference the low-tech solution has made! It's easy to start (!), obvious when it's running and/or running out and does not disrupt the flow of conversations. Everyone I have spoken to has agreed that it has saved time. It has not always worked, but in general the signs are positive so far. I have even seen the timer being used in other entirely unrelated meetings.