Open Source stock portal app Question

[Daniil Kulchenko]

Hey everybody,

I'm working on an open source stock simulation web app. I plan to have a
version available for a small amount of users (where accounts are
administered by 1 person), and a portal version (where lots of people can
register themselves).

I plan to have the portal version as one of my websites, so that people can
come register and try it out, so my question is: *how can I prevent people
from* taking the portal code, and making their own stock portals,* then
charging people to use it?* Is licensing the portal code under the GPL
enough, or do I need to do something else? Basically, I don't want (for
example) Yahoo taking my code, and making a product out of it without my
permission.

Thanks in advance,
Daniil K.
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