You might be able to just save the game, and load it changing the teams. I know you can change difficulty level this way on AIs.
Otherwise, you can leave teams unlocked and use the Dev.exe to change to the AI and break the alliance.
Under normal circumstances, the only way to be certain the AI will break an alliance is when allied victory is off, and the AI is allied with all remaining players. Only one player can win the game, so it has to break the alliance at that point.