All they need to do is mark the memo line of their check with the department they want their money to be spent and the church, by Federal Law, can't diver that money to other purposes.
So, for example, let's say you gave the church a check for $250. You can put on the note accompanying the check that you want $100 to go to the Sunday School, $100 to go to the janitorial budget and $50 to go to the choir (or any other places depending on your church's activities) and by law, they have to make sure that money is only spent in those areas you specify. You don't have to totally boycott your liberal church to prevent your money from going to the WCC.
All they REALLY need to do is get out of that church (and I use that expression loosely) just as fast as they can...
That doesn't always work. We learned the hard way. We just left our Church because money we had earmarked for our Sunday School Class was then diverted to a NCC (WCC is parent to) sponsored charity. Yep, we are looking for a new church and this worldly religion is the prime reason.
Then they just take someone else's contribution that would have gone to the janitorial budget and Sunday school, and send it to the WCC.
Yes, you can allocate which departments in your local church get "your" money. However, all funds become fungible when received by the church treasurer and even though "your money" will not go the WCC, the money from other members in your congregation will go to the WCC.
It requires an effort by church members to demand their local church stop sending any money to their denomination.