$ cat TAG
MYTAG (
Line 1
Line 2
Line 3
);
NOTMYTAG (
foo
bat
);
MYTAG (
Line 1
Line 2
Line 3 
);

$ perl -n -e 'BEGIN{$com=""}' -e '$com="#"if/^MYTAG\W/;print"$com$_";$com=""if/^\);$/' <TAG 
#MYTAG (
#Line 1
#Line 2
#Line 3
#);
NOTMYTAG (
foo
bat
);
#MYTAG (
#Line 1
#Line 2
#Line 3 
#);