I tried to use the solution 6 from here, but not exactly sure how to do it.
Prevent Spam Account Registration Solution 6: Update fields limitation rules from database : directly in the customer_eav_attribute table, update rows with attribute_id=5 [firstname] and attribute_id=7 [lastname] and replace 255 by 25 :
In my magento 2.2.6 version, the attribute_id=5 and 7 has
validate_rules="{"max_text_length":225,"min_text_length":1}"
and also
input_filter="trim"
I tried to change them to
validate_rules ="{"max_text_length":25,"min_text_length":1}"
and
input_filter =""
but i'm still getting a lot of Russian spam account registration.