They could, for example, commit to refusing to vote for any FF Government for 10 years.Barring a remarkable change of mindset among the electorate - unlikely, unless the economic crisis translates into a social/political crisis of unmanageable proportions - I see no way for Labour to resolve their dilemma. I think they are obliged to embrace it and settle for being occasional power-brokers within a political system that is profoundly resistant to radical change.
We would almost certainly end up with a minority FG administration, but one which had a certain confidence that Labour would not help FF back into power. Labour would be in a position of extraordinary leverage over the minority administration and be able to get some of their policies implemented purely by influence without formally joining coalition. While being free to tussle with the FF rump for the clear leadership of the Opposition. FF frozen out of power for a guaranteed decade minimum would have no point in existing.
Hey presto, clear left/right divide and the end of Civil War politics inside a decade. Result all round.