etp-wc-conditionally-unset-payment-option.php

<?php
/**
 * Unset the cheque payment option if any of the existing cart items happen to
 * be a ticket for an event which belongs to a specific category.
 * 
 * @param array $available_gateways
 *
 * @return array
 */
function maybe_unset_gateway_by_category( $available_gateways ) {
	$watch_list = array( 123, 456, 789 );
	$etp_wc = Tribe__Tickets_Plus__Commerce__WooCommerce__Main::get_instance();

	foreach ( wc()->cart->cart_contents as $key => $values ) {
		$event = $etp_wc->get_event_for_ticket( $values['product_id'] );

		if ( ! $event ) {
			continue;
		}

		$event_categories = tribe_get_event_cat_ids( $event->ID );

		foreach ( $event_categories as $category_id ) {
			if ( in_array( $category_id, $watch_list ) ) {
				unset( $available_gateways['cheque'] );
				break 2;
			}
		}
	}
	
	return $available_gateways;
}

add_filter( 'woocommerce_available_payment_gateways', 'maybe_unset_gateway_by_category' );