I can tell you how streemian.com will tackle the problem: It requires "verification" from the platform we are linking. 

### Reddit
People that want to link their reddit account need to authenticated against reddit using Oauth. Only after successful verification will streemian.com link the account and auto cross-post content

### RSS Blogs
Later this month, streemian.com will allow people to have the content of their blog cross posted on Steem using RSS feeds.
To verify ownership of an account, streemian.com will require users to post a specific token in a post and also need to verify a mail address (not required for reddit). Only after those two verification steps have been completed, will streemian.com auto-cross-post content from RSS-enable feeds to Steem.

**NOTE**, all content that is crossposted by streemian by a verified account will be posted under an account we will create for that specific user. That way, the content owner gets his reward while Streemian.com just simplifies the process for them.