To remove tags from items, use the dtag method. There are two forms:
$canvas->dtag(tag); $canvas->dtag(tag/id, deltag);
The first one searches for items with the specified tag and then deletes the tag. The second searches for items that match the tag or ID and then deletes the deltag (if it exists) from that item. This allows you to delete a subset of the tags, rather than every single tag.