Restrict Patent Ownership
There should be a limit on the number of patents owned by a single entity:
Real Human Person - 10 patents
Corporation - 1 patent per employee, maximum 1000
Any entity at the limit and wishing to obtain another patent must either SELL (not license) a patent, or file an Abandonment to Public Domain of a patent.
Patent trolls are typically 3 lawyers and a secretary, having to pay 1000 employees would kill their business.
Since a very large portion of corporate patents are used defensively, to prevent someone else from getting a patent, filing and then abandoning would work as well.
The numbers given are a first guess, but probably pretty good.