Old versions of WP-BlackCheck

After some versions and fixed bugs of my WP-BlackCheck Plugin, I did a small research on currently active versions and found out that there are many old versions out there. Some of them even contain mistakes which are already fixed in current versions.

As WP-BlackCheck gets its reply from a central server, it would be easy denying every old version with a nice error message forcing the user to update. But is that the right way? Your opinions please.