Don't just boycott. Actively do things that cost them money.

If I have $100 to spend, steering that away from them only loses them $100 or so. But if you actively interfere with their income streams, you're not actually limited to the amount of your own spending power.

Report them for fines and penalties. Poison the data they collect on the world. Take advantage of the costly perks they offer (e.g., 100% return policies on open box items), clog up their customer support lines, exercise your consumer rights to your data or data deletion, target their loss leaders to cut into their profits, etc.