The easiest way is to change the incident number to be randomly generated instead of incremently.
Otherwise in CRM 2013 you can utilize real time workflows to generate your own incidentID.. A guide for this can be found on http://niiranen.eu/crm/2013/12/auto-numbering-workflows-real-time-vs-asynchronous/
Rune Daub Senior Consultant - Dynateam CRM http://www.xrmmanagement.com