You can find the answer in the TortoiseSVN manual and SVNBook.
TortoiseSVN Manual tells us:
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. TortoiseSVN →
Unversion and add to ignore list 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.
See the important note from SVNBook; it explains the behavior and "why it does not work" in your case.
Subversion's support for ignorable file patterns extends only to the
one-time process of adding unversioned files and directories to
version control. Once an object is under Subversion's control, the
ignore pattern mechanisms no longer apply to it. In other words, don't
expect Subversion to avoid committing changes you've made to a
versioned file simply because that file's name matches an ignore
pattern—Subversion always notices all of its versioned objects.