Hi Brandon -
When you say "delete" do you mean removing control from enrolled devices in bulk or just removing device records from the portal for devices no longer enrolled?
If it's the latter, you could group devices together by a common thread (usually with old device records it's easiest to do so by "Last Checked In More Than XX Days Ago" (for instructions on creating device groups: https://www.ibm.com/support/knowledgecenter/en/SS8H2S/com.ibm.mc.doc/pag_source/tasks/pag_devices_searching.htm)Once the group of these old device records is created, navigate to Devices > Groups, find the group name, hover over "More" underneath the name, and select "Hide." You'll have the option to make this apply to all new devices that populate the group. This means that once a day, the group will check for new devices that have not reported in XX amount of days and automatically hide them for you.
Hope this helps