Check out the `evaporate' property of overlays.
You could set it to t so that your overlays get automatically
deleted when their size gets down to 0.
Great. Many thanks for the information. I
will be able to produce a better
support for Emacs. The new version is available at:
http://www-sop.inria.fr/mimosa/personnel/Manuel.Serrano/flyspell/flyspell...
--
Manuel