How to deal with copyright violations in our OSM data?

It is a good idea to first discuss the matter with other mappers in the region - an extra pair of eyes cannot hurt.

If you determine that this indeed looks like a copyright violation, you should first contact the user who has uploaded the data. In doing so, always assume good faith; it is possible that a free source of data exists which has been used by the mapper in question and by the copyrighted source you are comparing to, leading to both looking the same. It is also possible that the user has received special permission to upload the data to OSM.

Instead of "STOP UPLOADING COPYRIGTHED DATA!!!!" write to them something along the lines of: "Dear user, some recent edits of yours in the XYZ area bear a striking resemblance to the Google map. I am sure you are aware that we must not add data to OpenStreetMap without the consent of the rights owner, and we are not allowed to use Google maps. To avoid questions from other mappers and users, could you add a "source" tag to your changeset or describe the data source in the changeset comment? And can you tell me where the data has come from, because I would sure like to use that source myself if it is legal! If you should have mistakenly uploaded copyrighted data, then simply remove it. I can help with that if needed."

If the user replies and it turns out that he was indeed copying data, and he now wants to remove it, then offer him your help or find someone who can.

If the user does not reply within a reasonable time (take into account that they might be on vacation or not checking their mail every day), or refuses to cooperate, you can take the matter to the OSMF data working group by emailing data@osmfoundation.org. Be sure to include all necessary information; the data working group does not have access to your Inbox/Outbox so you will have to copy any relevant communication with the user.