In case someone else comes to this thread looking for this answer, I'll clarify the previous post:

It is NOT necessary to do hex editing to disable smileys.

It IS possible to disable smileys completely and just show the actual characters as text.

To do so, gain root access (look for guides on this if you aren't sure) and type the following commands:

The first two commands rename the smilies definition file so that the Conversations application doesn't find it, and the third restarts the Conversations application so the changes take effect (any active SMS or IM windows will be closed).

To restore smilies, execute the same commands, only in the second one switch around default.def and default.def.dist. Or, if you want to disable only some of the smilies, then instead of renaming the file, edit it and remove or change the lines you don't like.