Windows Server 2012 R2 safe to delete Address Leases

if they were not reserved and have not been connected for a long time and currently not being used, you could delete them though the configuration of the scope should be reviewed; sounds like you have a very long lease time that could be reduced so the address will be available once it is no longer in use by that device

Thanks Seth, I am not sure how I can tell what the Lease time is set on the server, however perhaps my last question related to this would be, would those Leases prohibit any new devices that come onto the network from picking up the IP which is leased but currently not active?  

right click on the scope and go to properties; the lease time is there
if an address was specified for a device and is no longer used and the least time has not expired, no it will not be available for something else to use

would those Leases prohibit any new devices that come onto the network from picking up the IP which is leased but currently not active?

Yes. It might get to the point that no IP address will be available so no new device will get an IP address.

if i deleted all leased IP addresses that are not active and not reserved would this be good then?

Yes. If these devices don't appear in the network anymore, delete these leases. You don't need them.