An undivided interest is a fractional ownership without physical division. In other words, the common elements are those items outside the individual unit/s. Land, hallways, lobby, exterior walls, roof, and pipes in the walls are all examples of common elements. You cannot divide the hallway or lobby or land and say "This part is mine, keep off."
Your board needs to contact an attorney familiar with condominium issues. This owner may be violating your board's rules, regulations, and by-laws. It is possible he could be fined for his actions and if he fails to pay up a lien is placed on his unit and ultimately foreclosed. That's right, foreclose on him and force him out of the building.
Your board must take action, failing to do so could place the board in jeopardy for failing to fulfill its fiduciary responsibilities.