There is really no way to automatically make the next number 100 to replace the deleted item. Once a number is used by Service Desk, it is never automatically recycled even if the item is deleted.
Service Desk doesn't keep track of numbers it can use for a module. Instead, it keeps track of the next number in sequence and uses that number for any new items (IFC_SEQUENCES, I think). I would not recommend resetting the sequence number as a work around, as that could produce duplicate Service Calls numbers for subsequent items.