Emoticons are ok, when used sparingly, or not talking to your boss.
The last thing you need in the latter situation is for your conversation to be littered with
or or even worse
To disable them, first click the icon circled in red below.
Alternatively, you can press the Alt key.
Either way, you need to select Tools –> Options.
In the resulting dialog, you need to select Messages, then untick Show emoticons.