Dear Mr @ABraun,
I have performed further tests, and i can say that Erosion and Minimum filters are not the appropriate filters to use in order to achieve my goal, that is to eliminate the single isolated pixel extracted (to respect a concept of Minimum Mapping Units … I can’t pretend to extract flood information from a single-pixel!).
Minimum and Erosion filters have the effect to delete randomly information (also the good one).
In any case, your suggestion to set No-Data Value has been fundamental to guarantee the correct run of filter algorithms.
Many many thanks!