Do like Wisconsin did under Republican gov Scott Walker: prohibit making paying union dues a condition of employment.
When Walker got the state to pass this one thing, where unions now had to get teachers to send them checks for dues, teachers left the unions.
Same in Indiana, but we did it differently. We gave teachers protection against lawsuits. Many teachers joined the union for the free legal services. When the State passed a law providing state funded legal services if a teacher was sued, membership dropped rapidly.