Capitalism allows unlimited private property with all the consequences for concentration of capital. If you don't want that you have to let go of something. Approaches that come to mind (and have been suggested) are

• taxes on capital (which basically takes property away)
• taxes on on income that approach 100% and thus limit income
• limits for incorporation (anti-trust laws)
• contracts that structure how income can be used (e.g. trusts are often highly structured in their spending)

Basically I'm suggesting very general contracts that effectively change the linear reward effect you mention first to a consistent sub-linear one.