Question: Some people erased three author names including mine from a paper previously submitted (and rejected by three different journals) and got it accepted by Case Reports In Emergency Medicine. 

I presented the publishing editor of the journal multiple clear proofs of the plagiarism. Although the journal declares that it complies with COPE, the publishing editor is dragging feet not to publish a retraction about this article despite the proofs. Instead, they are trying to overlook the situation by involving the politically driven hospital administration from where the paper was submitted from. This in turn caused violation of my rights both as a human and as a researcher. I am currently subjected to serious mobbing by the hospital administration secondary to the inconclusive behaviour of the journal. This mobbing includes the change in my workplace and an official warning because "I ruined reputation of the hospital by contacting the publishing editor".

I also sent a message to COPE but there is no response. 

Would anyone suggest any further action?

Answer: 
Working with the editor, until that process is finished, is probably the only thing to do at first. It will undoubtedly be a long process, with delays, so patience and persistence will be necessary. Try not to let the mobbing get to you, and try to remain calm and professional at all times, despite any pressure others may exert. 

If the editors, in the end, are unable to come to a resolution you accept, then the options will be (essentially) to escalate the situation or to let it go. But making an action of that kind while the editors are still working would be premature, in my opinion.
Search for legal help from an expert in I.P. , expose them publicly and hope for the best.
This doesn't mean that you'll be able to prove the rights if you didn't took enough precautions to protect your paper and ignorance by this side is hard to prove in court.
The next time though, you can publish your own thesis/essay/book and get an ISBN with total rights and you might still be able to publish your work before it gets published since "accepted" doesn't exactly mean "released" or "printed"(this isn't clear in the question).
After years of oppression, I find this as the only way to get proper recognition since many of those cheap "book makers with ISBN" can print your work "in hours" and you can begin to spread your own knowledge at your own pace and people will think twice before abusing with your intellectual property.