Setup correctly, an action hook or filter defined in a class is as easy to remove as any other hook. But if not setup correctly and not your code, try this.

remove_filter_in_class.php

<?php
function remove_filter_in_class( $tag, $function_to_remove, $priority, $class ) {
	if ( isset( $GLOBALS['wp_filter'][ $tag ][ $priority ] ) ) {
		foreach ( $GLOBALS['wp_filter'][ $tag ][ $priority ] as $key => $function ) {
			if ( is_array( $function['function'] )
				&& $function_to_remove === array_pop( $function['function'] )
				&& $class === get_class( array_pop( $function['function'] ) )
			) {
				return remove_filter( $tag, $key, $priority );
			}
		}
	}
}