we're using 7.6.
In ProspectLists we have a special list type 'do_not_delete'. With this list_type, it shouldn't be able in the frontend to delete an entry from Contact and Lead subpanel.
I tried a lot in costom custom/modules/Contacts/clients/base/views/subpanel-list/subpanel-list.js, but the delete button is not removable. It seems that the parent information comes to late. A script like https://community.sugarcrm.com/thread/27210 does not work.
The best solution would be to remove "delete row" from subpanel list, but I'm open for any other ideas, e.g. changing the unlink_confirmation overlay or manipulating the rowactions in viewdefs with PHP.