If you don't want someone else to be able to build a competitor site, it sounds like you don't want an open-source license. The best you can do with an open-so

If you don't want someone else to be able to build a competitor site, it sounds like you don't want an open-source license. The best you can do with an open-source license is the AGPL, which will force any competitor site to release all of their modifications -- this means that they can't gain any advantage over you, because you'll have access to all of their code as soon as they deploy it.