su -c "kill $(pgrep mdnsd)"
^      ^    ^ ^
|      |    | |
|      |    | +- Search the process list for the phrase "mdnsd" and return the process id
|      |    |
|      |    +- Spawn a subshell and execute contained command and return its output
|      |
|      +- Stop the process with the process id specified after kill
+- run the command specified after -c with superuser privileges

# stop mdnsd on startup
on property:sys.boot_completed=1
    stop mdnsd

# stop mdnsd when adbd starts/stops
on property:init.svc.adbd=*
    stop mdnsd

~# setprop ctl.stop mdnsd

~# chmod a-x /system/bin/mdnsd