When a single issue board member dominates board meetings, what can be done?
This answer depends on when this person is raising these issues and how your agenda is structured.
If your board member is speaking to an agenda item approved by the board and there is a motion and second on the floor regarding that agenda item, then it is appropriate to allow comments as long as the board member is not attacking or criticizing an individual. Although other board members may not agree with or want to listen to those comments, the board member does have a right to have their make statements related to the motion on the floor. If the comments go on extensively it is appropriate for the chair to ask them to summarize, and point out if they are repeating themselves. If the comments are not relevant to the motion then it is appropriate for any board member to call a point of order and ask to have the comments limited to the motion on the floor. A point of order can be requested as an interruption to the speaker, although it is best to wait until they take a breath. A board member may also make a motion to "call the question" and end debate. The motion to end debate is intended to be made at the end of the last speaker's statement and requires a vote of 2/3 to approve.
If the board member is raising issues not on the agenda, the chair needs to end the discussion and insist that topics only be taken up and discussed if they are listed on the agenda, and then only after a motion and second have been made. This practice ensures that at least two people on the board want to have a discussion of the motion (motion and second), and that a majority of the board want to bring up the topic (approval of the agenda by a majority of the board).
Some boards have an agenda item called "Board member comments" typically at the end of the agenda. During this time individual board members can raise any topics they wish. Most experienced board members and administrators regard this as a less than optimal agenda item for exactly the situation you describe. If this is when these topics are raised your board may want to reconsider this as an agenda item. Board meetings should only cover topics a majority of board members wish to discuss. Ideally the public and the administration should be fully informed in advance - `No surprises' - by listing items of discussion on the agenda. An open item like "Board member comments" opens the door to unpleasant and unwelcome surprises.