If you have not yet committed the add, then all you have to do is use→ to undo the add. You should then add the file(s) to the ignore list so they don't get added again later by mistake.
If the files are already in the repository, they have to be deleted from the repository and added to the ignore list. Fortunately TortoiseSVN has a convenient shortcut for doing this.→ will first mark the file/folder for deletion from the repository, keeping the local copy. It also adds this item to the ignore list so that it will not be added back into Subversion again by mistake. Once this is done you just need to commit the parent folder.