Link to the mod (https://custom.simplemachines.org/index.php?mod=4292)
What does it do?
This mod will check new registrations against a list of known proxy headers. If any of the headers were sent with the request, the registration will be logged in your Admin Panel->Members->Possible Proxy Registrations. Deleting a log there will not delete the member btw, only the selected log(s). Should you wish to delete a member who registered through a possible proxy server, you can do so at the View All Members tab.
As mention it checks only new registrations. But it can be modified to check log ins as well, should there be enough interest in this.
You must take into consideration that not all proxies send headers that identify themselves as such btw.
Currently the mod is compatible with Smf 2.1. rc versions only. But it can be made compatible with 2.0.x versions as well, should there be enough interest in this.
This mod is fully supported. Should you have any questions or need any help with it, please post at its own support topic.
This mod is released under Mozilla Public License. Enjoy it :)