How do these non extension files get generated and how to prevent this?

The files are not extension-less, you just don't have extensions shown in this folder.  You can see that with the word docs - there is no extension listed, but they're type is Word.  The extension is either .bestand or is something else but Windows knows they belong to a program called bestand.

"Bestand" is the Dutch term for "File" used in Windows - it's not the associated program. It just shows that Windows doesn't have a file association registered - that could be because there is no file extension, but because the user setting for "hide extensions for known file types" is on by default (shown by the missing extension for the PDF).

We don't know which folder this is, but by design lots of programs will create temp files with this format including AV and software firewalls.  

Try arranging the folder by file type and deleting all of them - see if any are locked because the creator program is still using them (skip delete for those) then watch the folder to see how quickly they are recreated.  For those files that are locked it's possible to identify the process and so the program that is using them.