look dont need the if $1 == !calc, cause if you put an space here

on *:text:!calc *:#: {

this

!calc123

will not happen, by the way, why dont u do a check for

if ($2 isnum) 

to check if $2- is number, then you will send a message to channel like

if ($2 !isnum) { msg $chan ERROR: syntax: !calc 1+1 }

the $strip identifier, strip all incoming colors