Excluding emergencies, have students write down issues with others and place them in a box at your desk or some other central location in the room. It is vital, however, that you clearly explain what is and what is not an emergency. For instance, if physical harassment or bullying is occurring, then that needs to obviously be addressed immediately. Regarding everything else, let the students know that written complaints will all be read and dealt with if need be.
Try scoping out the cause of the tattling and nip it in the bud. For example, if a student is constantly getting distracted for whatever reason and subsequently being told on by his neighbor, focus on the causes of the distraction instead. Basically, treat more than just the symptoms and address the bigger problem. Not only will the student learn better, but the tattletale will no longer have any reason to bother you.
By clearly defining what is right and wrong, you will help eliminate frivolous tattling. For instance, when a child feels compelled to tell on someone, they should have a reference to turn to for determining the legitimacy of the would be complaint. For some older students, this may be something as simple as their memory of what you have instilled in them. But for younger ones, have plenty of posters and signage in place to reinforce what is and is not unacceptable behavior.
For serious matters, it is a given that you should be the mediator and disciplinarian. However, for petty issues related to tattling, encourage the students to work things out themselves, as it is important for students to learn how to manage conflict and resolution as students' minds grow and develop. In fact, make it a rule: no non-emergency complaints will be listened to unless students have attempted to first work things out themselves.