If they are somebody new, I think we should greet them as normal. For me that usually means seeing Praise's note that she has e-mailed them inviting them back to the community.

if the ad looks interesting, comment on it as normally as we ever do anything.

if they spam us, politely explain "don't do that".

or ignore it.

in other words, business as usual, unless the person is obviously spamming the forum.