First notify EMC that you are decommissioning your array. If you are using ECC to notify EMC, you will need to go into Administration > Alert Definitions > Storage Agent for Symmetrix > alarms > comm,device,dir,disk ..etc  ..right click on alerts that are enabled (red color) > edit Alert > Apply to tab ..and remove the Symm you are decommissioning.  Do it for all alerts, there might be another way to do it for all alerts at once but I have not found it.
we are going to decomm a dmx array. 
Disable alerting/monitoring on a Symmetrix array?