The risk of plagiarism is minimal since plagiarists rarely get away with such theft.
If you are concerned, here are several steps you can take to minimize the risk of someone stealing your material:
1) Allow only those who critique to view your story. This is an option you can tick when you send in your story and will lessen the number of people who read your story. However, you may receive fewer crits since new members won't be able to look at your material.
2) Hide your story once the critique period is over — that way nobody else will be able to look at it. View your story under "My Page" and click "hide story". Again, this means people won't be able to critique your story after the one week period has passed, and you may therefore receive fewer crits.
3) Delete your story.
IP numbers are registered, as is who reads which story, and in case of actual cases of plagiarism this information may be used in an attempt to track down the culprit.