Woo hoo, I had this problem, and TortoiseSVN has supported solutions for both add and delete an item from the sparse checkout.
http://tortoisesvn.net/docs/release/TortoiseSVN_en/tsvn-dug-checkout.html
1.To add:
In windows explorer, Right click on the checked out folder, then use TortoiseSVN ? Repo-Browser to bring up the repository browser. Find the sub-folder you would like to add to your working copy, then use Context Menu ? Update item to revision….
2.To delete;
From your root, right-click / Repo-Browser / Update item to revision; then select ‘Exclude’.