In Hilchot L"H 9:2, the Chafetz Chaim states this prohibition, but
qualifies it in two ways:
(1) If the speaker knows that the listeners will not speak negatively
about the subject it is permissible to praise him, just not excessively.
Presumably those who gathered for a celebration related to the subject are
not likely to _speak_ against him (even if they think about some incident
regarding the subject).
(2) If the subject is known by all to be a good and righteous person in
whom no guilt is found. In such a case, anyone who would speak against
that person would be ignored.
But this source does seem to imply that a speaker should be careful not to
praise someone who is not part of the program, which could apply to
synagogue announcements, sermons, and other gatherings which are not for
one particular person or group of people.