Pushing these companies to return money to their shareholders instead of keeping it in huge war chests used for buying up any potential competitors would be a good first step. Maybe something along the lines of "any company whose average market cap over the course of a fiscal year exceeds 1% of GDP is automatically broken up". Companies that don't want to be split into pieces would need to find ways to reduce their market cap, one of the quickest being sending dividends to shareholders, who could then invest that money into other parts of the economy or even other potential competitors.