Mitzvah objects such as a shofar, daled minim or tzitzis which are still functional for the mitzvah may not be used for any other purpose. Doing so would be a bizyon hamitzvah. Therefore, tzitzis strings which are still affixed to a tallis may not be used to secure any objects. This prohibition extends past the time of the fulfillment of the mitzvah-- i.e., it also applies to using tzitzis at night, or the lulav and shofar after Yom Tov. Objects used for a mitzvah mid’rabonon may not be used for mundane purposes either.
[שו"ע כא, א, ומשנ"ב ד; וראה ביה"ל ד"ה אבל]