You cannot opt a paper out of the plagiarism check. However, it is possible for the author of a paper to define whether Urkund is allowed to check if material in the uploaded paper is used for plagiarism elsewhere. That is, if passages of text have been copied passages from one's response into new responses submitted by anyone other than oneself.
If you do not want assurance that your paper cannot be plagiarized, you can opt-out it out of the document pool on the flow page of the specific exam, the day after the submission deadline. In some cases, you will also receive a receipt directly from Urkund where the possibility to opt-out is also available.