the syntax is:

access-list <acl_name> deny tcp any any gt 1024

be very careful though. Many applications will legitimately use ports higher than 1024. My advice would be to monitor traffic through the PIX before deciding to use a rule like this. THe best solution would be to look at some type of content filtering solution like Websense. Many of these will allow you block IM traffic based on the database that is updated from the vendor. It is much easier than trying to do it manually. Also, a proxy server for outbound web traffic would allow you to have much greater control over traffic leaving your network because you could restrict all ports on your PIX except the web traffic coming directly from your proxy . Beware though, many IM programs now use HTTP an HTTP transport which can use a proxy. But, that is why I recommend using content filtering.