Yes, they do use specific ports, and if those ports are blocked then they will route through port 80 (ICQ also uses 21). Port 80 is used to access the internet via http. I can't really block that.
MSNM for example defaults to 1863(9). That port is blocked....it will then go through port 80.

MSNM, ICQ,and Yahoo (pretty sure about yahoo) all re-route through port 80 if their original one is blocked.

So blocking their specific ports will not help.