Prevent plagiarism after rejection of a paper? After reading the question What to do if a referee plagiarises the result after rejecting a paper?, I wonder what can be done to prevent this.

I also recently got a conference paper rejected with a good (but not very good) idea. Reviewers gave me 2 × weak accept and 1 × accept and only the meta-reviewer rejected the paper with bogus arguments.

The next conference where I would like to submit is in a few month such that nearly half a year is in between the two submission deadlines of the conferences. I don’t know how may people had access to my work and would like to protect my idea.

I can maybe upload it to arXiv now before submitting it to the conference. Some conferences are fine with this as for example ICML: 


  Publication at http://arxiv.org explicitly does not conflict with
  ICML.


but I am afraid that this can become problematic for other conferences.

What else can I do?
I think that this has been covered on this site before.  But briefly:


Most academics (in particular those who have weighted in on the issue on this site) agree that the best way to prevent plagiarism is through more light, not more secrecy.  In other words, once you've shown your work to one other person (and in many academic fields, this will necessarily occur in the course of doing academic work), the more people who know about it -- and the more that people know that people know about it, and so forth -- the less likely it is for your work to be plagiarized.  Thus posting your paper on the arxiv would be an ideal solution (and have many other benefits as well) unless you're working in an academic field which has the practice of regarding, or potentially regarding arxiv submissions as "prior publication".  (If so: have you considered switching to math / physics / TCS instead?  In those fields we not only do not feel that way, we regard the whole notion that freely showing your work to others counts as "publication" as being infinitely lame and wrong-headed.)
Even if you can't or don't want to upload a public copy of your work, you have already done plenty to establish your priority.  Namely you submitted it to a conference and got three referee reports.  You should certainly keep these reports for your records along with the correspondence with the handling editors.  If later on someone tries to steal your work, all you have to do is produce this correspondence.  You should also be able to count on the conference editors / organizers for help: whether the paper is accepted or not, if a conference submission results in someone's work being stolen, the organizers of the conference have an ethical obligation and a professional stake to be actively involved in your pursuit for academic justice.