If you are looking at the auto-generated rewrites in Magento's 'URL Rewrite Management' page then all of those URL re-writes are actually necessary in order for Magento to serve your products/categories at search engine friendly URL's.
If you wish to remove any custom / redundant URL rewrites, you can empty the
core_url_rewrite table in the database (using the below SQL query). Once emptied, Magento will reinstate all the necessary URL rewrites that the system requires based on your store configuration.
SET FOREIGN_KEY_CHECKS = 0;
TRUNCATE TABLE `core_url_rewrite`;
SET FOREIGN_KEY_CHECKS = 1;
You can not reduce the amount of auto-generated redirects that Magento requires as this is how the system is built. Instead, you should make use of canonical URL's in Magento to ensure that search engines only pay attention to you desired URL display on your website.