I doubt it's a conflict of interest as most groups would have married couples on thier boards.
Not a lot you can do, beyond engineer or let it blow up, like water it is a good leveler - then sort out the pieces later.
With 5 they can be still out voted. Loby everyone to  keep doing it & he will either get the message or spit the dummy altogether.
Unless he is totally thick?
No matter what you will loose 2 board members.
Even if you move for dismissal under 'Conduct unbecoming a member'.
Failing that wait out his tenure & stack candidates against him at the next election.

We've had 2 couples on our board of 9 but they never seem to vote as a block.  All 4 have their own opinions and all of us have enough "sand" not to be intimidated by anyone.   We all get along pretty well.  We're able to disagree without being disagreeable.   Gaafa's advice seems right on target.  You just need the stamina to get through to the end game.

We run a very democratic and fair board election system (even if they didn't vote me in this year).  If the members vote for individuals and if a married couple get elected then so be it.  We currently have two married couples, a mother/daughter team and three individuals and I feel they are doing their best to stay fair.

[ Theatre is Drama...most of the time it is at the meetings! [/QUOTE] I love this quote!   So true... 

Sooner or later everyone is going to have to face the music, no matter how cacophonous it is!  So, I agree with those who have suggested a face-to-face is necessary to discuss the situation with the perpetrators.  But how that meeting is structured and who attends it is quite critical.  I'll come back to that in a moment, but first, a couple of other points.

You have no rules governing couples on the board and I don't think that this is the problem, anyhow.  Such rules are double edge (pun not originally intended, but I'll accept it as fruedian) swords in that during the long and healthy existence of any organization, there may ordinarily be far more positive experiences than negative ones.  But nothing is so overwhelming than the problems of the present. 

Next, I think a five member board is too small.  I don't know the rest of your organizational structure, so it is difficult to suggest a more appropriate number, but with just five board members, even one disruptive member will have a way of creating problems with the functioning of the board.  In community theatre, I believe having a  counter-productive board member needs to be an expectation and organizational and board structure needs to reflect that expectation.  There is safety in numbers, so expanding your board would be a long-term solution to the problem. 

We have probably all had experiences with a belligerent  board member.   The most common way to deal with it is to tolerate it until the person goes away.  But that may not be the most effective way to handle it.   First, try to listen to the person's content and not the tone.  There may be legitimate issues or even a worthy idea being expressed.  Any face to face you have will then focus on the appropriate problem.  It could be as simple as "You have important things to say, but people are only hearing an attitude instead of the idea."  So often, this is exactly the case.  The more people ignore or brush off the content, the more belligerent the individual becomes feeling like they have to get more and more obnoxious in order to be heard.  And all they really want is to be heard.  Recognizing their content and letting them know how they come across can help the situation. 

On the other hand, there are people who have ideas that are contrary to the needs, desires, goals, vision, mission (I can make this list very long, so I'll stop here) of the organization.  If this is the problem, then the long term solution lies within your nominations process.  Unless there is a concerted effort by the broad membership to change the direction of the organization, prospective board members need to be committed to organizational priorities.  Short term, the discussion you have with the perpetrator has to do with clarifying organizational orientation and priorities.  If he/they disagree with that orientation, then you - and they - have a deeper set of issues to deal with.  How much support is within the entire organization for their ideas?  Maybe most people do agree with them and it's the rest of the board that is out of step.  It is a possibility.  On the other hand, if they are out of step, this needs to be brought to their attention.  But again, listen and consider the content of what they are saying.  There still might be a good idea buried in all that attitude.