How to comment at the end of a command in LTspice?
SPICE is not a well-standardized netlist description language. Every SPICE can introduce it's own syntax, special symbols, circuit elements, etc. Historically, only full-line comments were used ('*' in the first column), but there's no reason some particular SPICE might not allow ';' to introduce an end-of-line comment.

From the LTSpice Help file, under the heading "General Structure and Conventions':


  Any line starting with a "*" is a comment and is ignored. The line starting with "R1" declares that there is a 1K resistor connected between nodes n1 and n2. Note that the semicolon, ";", can be used to start a comment in the middle of a line.


So LTSpice does, indeed, allow end-of-line comments starting with a semicolon.