My content stolen- I get accused of plagiarism! need help and advice please.

Hi guys,
I submitted a few of my articles that I had published previously on my blog to helium.com last week. Yesterday I received an email telling me my account was banned because of plagiarism.

So, I looked into it and found out 3,4 different people have stolen my content and submitted on their site with no link back or credit to me and not even my name.One of them actually is doing it on article directory(greendoc.net) and in just a few seconds looking I found out he published over a dozen or so of my articles under his name.

So, I contacted helium and gave them the url and dates that shows those articles were published on my blog way before on these sites. So they are looking into it. Although I don't understand why they would ban your account before proving you are guilty. Yes I understand they dont have time to do this with hundreds of people. But they can at least give you a chance to prove your not guilty before they close your account.

Any way, I have a few questions that I am sure many experienced warrior can help me with.

1- I contacted these sites and asked them to remove the content immediately or else I will take legal action. Is there anything else I can do? If they decide not to remove, how do you prove it?

2- Is there any plug-in or a tool that can stop this?

3- Is here any tool to look for other possible stolen content?

4- Please feel free to give me any other suggestion or info you have, regarding how to keep it from happening or what to do to resolve faster or what have you.

Thank you very much in advance.