How can I disable scratch preview window?

This behavior is defined by the presence of preview in the value of the 'completeopt' option.

The default value is:

menu,preview

To remove preview, simply add this line to your ~/.vimrc or modify an existing completeopt line:

set completeopt-=preview

If you don't mind the preview window too much, but want to easily close it, you can use the :pclose command or CTRL-W z keyboard combination see :help :pclose.