This is a tough call. The Votto for Buerhle trade is probably fair to allow given that Votto hasn't played a full season in the big leagues yet and Buerhle is a proven commodity (albeit not a dominant fantasy pitcher). The Cabrera trade is another story.
My first bit of advice to you is if you smell collusion or some other form of chicanery, then your instincts are probably right. For that reason alone, any trade involving a team you suspect of collusion should be voided. I assume you know the person in your league who you suspect of collusion. You should think long and hard on whether you want to directly address the person or passively allow the transactions and risk damaging the integrity of your league.
While Miguel Cabrera has a long resume of fantasy baseball dominance, he is no Albert Pujols, Ryan Howard or Prince Fielder. That being said, trading him and Nunez for Konerko and Ibanez is not lopsided or unfair on its face. Is it a smart trade? Probably not. Konerko is on the downside of his career and cannot match the numbers put up by Cabrera. But Raul Ibanez, when healthy, puts up very solid fantasy numbers hitting in the middle of the Phillies' lineup. Combining the stats of Konerko and Ibanez (when healthy and productive) is easily equivalent to Cabrera. So I would allow the trade if it was made between two teams with credibility. The fact it is being made by someone you suspect of colluding or dumping the season is reason enough to nullify the trade and prevent him from dealing at all. If you need any assistance with handling this, email me at email@example.com