Boycotting is only one part of a three-part strategy that would be necessary if Americans are to cripple MLB. The other two parts:
1) the anti-trust exemption must be removed from MLB. This will not occur in the present Congress, so pressure would have to come from state legislatures, in a two-pronged process of passing a Constitutional amendment requiring a universal anti-trust addition to the interstate commerce clause, and in suing MLB to have its exemption removed by the courts. Both of these would take a while, but if handled correctly they could both tie up MLB in having to defend itself judicially and legislatively, hurting the teams in the pocketbook.
2) Cities and states would have to begin refusing to provide any funds or services to their MLB teams. This won’t happen in blue cities like NY, Boston, Chicago, or LA, but it could be used to cripple teams in red or purple areas that already have financial issues. To use a personal example, have Florida, along with the appropriate cities, refuse any further support for the Rays and the Marlins, allowing them to either go out of business or move elsewhere. If all red states gave up their MLB teams, the league would shrink both in number and in area—a blue-state-only league of 15 teams would be far less impressive than an all-country league of 30 teams.
This is war, in every way except blood. We should act accordingly.
If you want to talk about cities supporting their teams do not forget that Atlanta built two Stadiums for their team in real recent years and that should never be forgotten.