How to set a thumbnail without featured image?

Which theme is this regarding?
https://codex.www.ads-software.com/Forum_Welcome#Include_as_much_information_as_possible

customized.

so i guess i should contact the creator, right?

Yes, generally you can add conditional tags around the code that spits out the featured image, e.g.:

if (!is_page(42)) {
    // Do your featured code
}

great, thank you!