The ability to veto a trade is available in order to prevent collusion in a league. 
There are two ways to veto a trade, and in League Manager (Private) leagues, these settings can be changed by the LM at any time during the season:

Public leagues and/or Private league with standard settings: 

• EVERY team manager in the league has the power to cast a vote against a trade, EXCEPT the two teams involved in the trade. Once a trade is accepted, an email will be sent to every team manager in the league with detailed instructions on how to cast a vote. Follow those steps, and you'll be all set. 
• If enough votes are cast against the trade (4 out of 10, 50% of teams NOT involved in the trade), it will be canceled immediately. 
• Teams with more than one team manager will only receive one vote. 
• Once player(s) are traded, the players will be on the team's bench until the Team Manager manually moves them to the active lineup. 

Private leagues:

• Unless otherwise changed by the LM, all public league settings apply
• In LM leagues, LM has the ultimate power to veto a trade or let it go through
• LMs can see the number of votes for or against a trade before it is vetoed or processed
• If the rare scenario occurs where the setting is changed while trade is pending approval, that trade will be grandfathered to use the original setting in place when the trade was accepted
  Under LM Tools -> Edit League settings -> Transactions and Keepers