HeidiSQL only changing data, but not table structure?

Is there a way to use HeidiSQL as a data-tool only (without capability of any structural changes)? I would like to pass it to customers so that people can change their data, but they should be unable to do structural changes.

no.

You can go to "Tools-> User Manager" and create restricted users that don't have any privileges except "SELECT", "DELETE", "INSERT" and "UPDATE" on the desired database and/or tables. They'll still have the interface, but they won't be able to save changes. There' no guarantee it'll just work (you might need to tweak these privileges so HeidiSQL can do basic stuff like reading table names) but that's the closest you can get.

Said that, HeidiSQL is not designed as a data entry tool. There're endless solutions to build simple CRUD applications on top of an existing MySQL database (even Microsoft Access) and whatever business that spends moneny on data entry should probably consider an app as part of the basic investment.