Drupal allows you to alter a form using the hook_form_alter() hook. I work primarily in D7, but it looks like the same approach I'd take in 7 would work in 8/9.
You want to create a custom module and use hook_form_alter() to alter the validation part of the form. You want to check that whatever field you want to test against doesn't contain the characters you're getting.
The following is pseudo code, but hopefully can get you started in the right direction.
function mymodule_myform_form_alter(&$form, &$form_state, $form_id){
$form['#validate'][] = 'mymodule_myform_check_foreign_chars_validate';
}
// callback function
function mymodule_myform_check_foreign_chars_validate(&$form, &$form_state){
$char = "Д";
$mystring = $form_state['values']['textfieldnametocheck'];
// Test if text contains the word, character, etc, that you do not want
if(strpos($mystring, $char) !== false){
form_set_error('textfieldnametocheck', t('Certain characters not allowed.'));
}
The gist is you want to find out the $form_id of the form you want to alter. And, you want to add additional validation logic so that when the submit button is hit, your module applies that additional logic against the form validation process.
Above I'm testing for just one character, but you could create an array of characters to test against. It really doesn't matter what language it is, you just know the characters are not English or any language you're doing business in. You could add Chinese characters, Greek, etc should you get spam in other languages.
References:
How can I find the form ID of a form?
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21form.api.php/function/hook_form_alter/8.8.x
