We can perform this action through "Contact lost Model Destruction" and "Destruction delay(sec)". These settings can be found on the VNM Model, located in the Universe View. Simply highlight the VNM icon and go to it's Component Details view. Expand the Fault Isolation folder in the Component Details view and there you will see the Contact Lost Model Destruction and Destruction Delay options.
Here we can customize the Destruction delay in sec's which is the default value 604800 sec. which is equivalent to 7 days. If we want to increase the value we can do it through updating the value in Destruction Delay field which is under component detail ->information ->Fault isolation option.
After setting the value we have to Make it "Enable " in "Contact Lost Model Destruction ".
Let's take an example that if you fix it for one week and you enabled ,it counts the time from when you enabled it , and after 2 weeks spectrum will automatically remove the devices which are not responding to polls. and if you set the value for two weeks and spectrum will remove it after 4 weeks.