miniman If you have the default values in the config.php file for your Machform installation (if you don't know what this is, then the default values are most likely still there), then any fields you have deleted from the web interface are not gone; they are just hidden. Machform stores everything in an MySQL database and fields you "delete" are actually "flagged" to not be shown in the web interface. You can recover them if you can get access to view and edit the database. (Carefully!)
This is just the way Machform works, you can't prevent it from happening. Instead of deleting fields you don't want any more, hide them instead.
For help with this, your best bet is to open a support ticket with the Machform developers. You can only do this if you have an active support contract; if you don't have one at this time, consider purchasing one.