Try this:

In Windows firewall, make a firewall outgoing rule that blocks access of the following file:

C:\Program Files\Xara\Xara Designer Pro X\17\DesignerPro.exe

to IP address:

163.172.175.38

If you find this breaks something , then you can script the enabling and disabling of the rule as follows

Disable:

@echo off
netsh advfirewall firewall set rule name="{name of rule}" new enable=no
mshta vbscript:Execute("msgbox ""Firewall off. Designer Pro update check unblocked"":close")

Enable

@echo off
netsh advfirewall firewall set rule name="{name of rule}" new enable=yes
mshta vbscript:Execute("msgbox ""Firewall on. Designer Pro update check blocked"":close")

Now, this works for DPX16, but I will have to wait 12 months to see if it works for DPX17 but if someone who is getting the DPX17 nag screen might want to give this a try.