It is possible to block WordPress form submissions by a particular user agent by using the wsf_submit_field_validate filter hook.
An example filter hook function is shown below. This function will run for all forms on your form when they are submitted.
// My validation function function wsf_submit_block_user_agent( $field_error_action_array, $field_id, $field_value, $section_repeatable_index, $post_mode, $submit ) { // Only process validation if the form is submitted and not saved if ( $post_mode !== 'submit' ) { return $field_error_action_array; } // User agents to block $user_agents_to_block = array( 'Linux aarch64' ); // Check if $_SERVER and the User-Agent header exist if ( isset( $_SERVER ) && isset( $_SERVER[ 'HTTP_USER_AGENT' ] ) ) { // Get user agent $user_agent = $_SERVER[ 'HTTP_USER_AGENT' ]; // Check user agents to block foreach( $user_agents_to_block as $user_agent_block ) { // Check if user agent is present in the User-Agent string if ( stripos( $user_agent, $user_agent_block ) !== false ) { // Send a 403 Forbidden response and exit header( 'HTTP/1.1 403 Forbidden' ); exit; } } } // Return $field_error_action_array to WS Form return $field_error_action_array; } // Add a callback function for the wsf_submit_block_agent filter hook add_filter( 'wsf_submit_field_validate', 'wsf_submit_block_user_agent', 10, 6 );