I believe those "predictions" are actually what is called a "Value List". It is a feature in GDL: you define a parameter and you can also define a list or range of possible values for it. For Finishes in Zone Stamps, there is a default list of 4 possible values, plus the value CUSTOM (in which case you can define your own value). Those possible values are what you see when you click that right-pointing arrow in any of those cells.
You would have to modify the GDL Script of the Zone Stamp to modify that list or remove the possible values.
Value Lists are used extensively in the default ARCHICAD Library. For example there is a value list for Zone Names. Last year I wrote about how to modify the list of Zone Names available in Zone Stamps:
http://archicad-talk.graphisoft.com/viewtopic.php?p=257382#257382
If you know some GDL, you could do the same for Finish parameters and create you own value list for them. Then you would only have to select your choice from the list, not type in the field.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28