Manage Subscriptions ->Tags -> (select tags to delete) -> Delete selected
For the un-initiatied
Google Reader: is an application that can be used to store a collection of "RSS" feeds from a your favourite websites. Using these feeds Google Reader regularly searches and shows you any updates made to the website . So, you always know what is new in the website.
RSS Feeds: are similar (but not the same) to a common website address. But these are more "active" since these help readers (like google readers) search the website for latest updates. If you want to know the latest updates on, say, BBC and wondering where to get the RSS feed link, just go to BBC and search the page for "RSS" (usually at the bottom or top of the page with smaller font, like "contact us"). Click on it and it will lead to a separate page that has the RSS feed links for all the sections of BBC like news, podcasts etc.