All commands can be re-bound, though you have to follow a few simple rules.
First, to unbind somthing, it needs to have been bound in the first place. As such, the module that provides the commands needs to be pre-loaded.
Second, the unbind command, should look exactly how it would have been bound in the first place (had it been done in Tcl), with the "un" at the begining.
Too workout the unbind command, you use the following format
unbind <type> <flags> <keyword> <command>
This is a DCC command, so you use DCC for <type>
The flags need to be exactly how it is bound. Using ".bind dcc all" int eh partyline, will help you find the flags.
the keyword int hsi case is adduser
The command can be easily worked out. If it's provided by eggdrop, then the command will be in the following format
*<type>:<keyword>
Thus, for the adduser command, you use
*dcc:adduser
However, the simplest way to obtain it, is used in command above, to obtain the flags, as the output also shows the command name.
This is enough to get you going on this, but there is one small catch.
Some commands, are either bound to all users, or very low level flags. Within the code, there will usualy be a second flag matching sequance.
Thus depending on which way you change the flags, it still may not work as you plan.