When I do this, the column is no longer displayed in SoSci. However, when I restore the item in the question, the data appears again.
This shall not be the case. If you created a new item in a question, it should get a new variable internally. The data previously collected should not appear in the dataset, again.
In general, SoSci Survey won't touch the data already collected. If you delete the question or item, the variable will disappear, but the data is still in the database. You can also download this data, if you select "selected variables" in the "variables" tab in the download settings.
There is no way to delete variables from the database, you can only delete cases. So, if you have reason to remove data from the database, download the data, make sure everything is there, make a backup, and then delete all collected data from SoSci Survey.