The constant problem with guestbooks is that one man's spam is another man's treasure, and always causes issues amongst the staff in HT.
In completely ignoring this "feature", the BBs have removed this conflict of opinions. (Fair play to them).
If you want to congratulate people, you could always send them an individual message, or is that too personal? (My cynicism lies in the point of why do people feel the need to see their comments posted on other team pages when a perfectly good messaging service is in existence... there must be some reason, however bad or good, for people to post about this).