This snippet removes the default patterns and "blocksy" pattern category created by blocksy theme

remove-blocksy-patterns.php

<?php
add_action("init", "init_blocks_and_patterns",100);
function init_blocks_and_patterns(){
	if(\WP_Block_Pattern_Categories_Registry::get_instance()->is_registered("blocksy")){
		unregister_block_pattern_category( 'blocksy');
		$blocksy_patterns = [
			"posts-layout-1",
			"posts-layout-2",
			"posts-layout-3",
			"posts-layout-4",
			"posts-layout-5",
			"tax-layout-1",
			"tax-layout-2",
			"tax-layout-3",
			"tax-layout-4"
		];
		foreach($blocksy_patterns as $pattern){
			if(\WP_Block_Patterns_Registry::get_instance()->is_registered("blocksy/".$pattern))
				unregister_block_pattern( "blocksy/".$pattern);
		}
	}
}