I'll echo horrorfan to a point and ask why is the rule necessary? It seems to me that the 2 guys carrying minimal D-men are trying a different strategy and the results appear to show it works. Guys try different strategies all the time: goalies early in the draft, goalies late, premium D-men, goon players etc. etc. etc. Personally I think its up to each individual owner to try and decide which strategy they think will work. I don't see what this rule does for the league.
If you had tried to implement the rule after the draft I would say you have no leg to stand on but given that you gave them 3 days I think this is reasonable for them to realize they need to draft 3 D and adjust accordingly. As an aside did all managers confirm they were aware of this rule?? If they weren't then your case is weaker.
I don't know if yahoo has anyway to penalize "illegal rosters" otherwise you may have to manually penalize them games (did you establish what the penalty is for an "illegal roster"? without that in place its hard to they knew the penalty in advance.)
All in all I'd also scrap the rule for this season. Have a discussion in the offseason about why you think the rule is necessary (I assume you want 36 D-man active, makes D more valuable, something like this) and what the penalty will be for an "illegal roster" and whose obligation it is to report an "illegal roster". Then I'd put it to a league wide vote requiring 2/3's to carry.
12 team H-2-H 1 year league, daily roster changes, 3 goalie start minimum/week
2xC, 2xRW, 2xLW, 4xD, 3xUtil, 2xG, 5 Bench
G, A, P, +/-, PIM, PPP, SHP, GWG, Hits, W, SV%, GAA
C: Malkin, Backstrom, Stepan, Ribeiro,
LW: Vanek, Moulson, Dubinsky
RW: Kessel, Vrbata, Clutterbuck, Okposo, Setoguchi,
D: Green, Johnson, Wideman, Bogosian, Gudas
G: Lehtonen, Holtby, Mason