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, this settings can be changed by the LM at anytime 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), then it will be immediately canceled. 
• Teams with more than one team manager will only receive one vote. 
• Once player(s) are traded, the players will be on team's bench until Team Manager manually move them to 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 are able to see the number of votes for or against a trade, unless LM is part of a accepted trade. 
• PLEASE NOTE: In Weekly-Lock leagues, if LM upholds a trade in the middle of the week, the players along with their stats will follow to the new team IMMEDIATELY. 
  
  For example, if LM executes a trade (Team W sends Shohei Ohtani to Team E for Anthony Rizzo) at 10pm ET on a Friday, Shohei Ohtani's stats from that Monday to Friday will be with Team E the moment the trade is upheld. 
  
  
• PLEASE NOTE: In Daily-Lock leagues, if LM upholds a trade during the day (before 3am ET everyday), the players along with their stats will follow to the new team for that day.
  
  For example, if LM executes a trade (Team A sends Mike Trout to Team B for Mookie Betts) at 11pm ET on a Thursday after all the game ended, Mookie Betts' stats will end up on Team A for Thursday. 
  
  
• In the rare scenario occurs where "votes required to veto" settings is changed while a trade is pending approval, that trade will be grandfathered in to use the original settings that was in place when the trade was accepted
  Under LM Tools -> Edit League settings -> Transactions and Keepers