Disabling the popup by crippling the executable is discouraged when there's a package manager. You don't know what else will break and updates may eventually reintroduce the file when you least expect it.
Instead you should find the package providing captive-login and examine what other files it provides, usually with Debian derived systems you'll find that large software suites are split into several smaller packages.
You can find which package captive-login belongs to with
dpkg -S $(which captive-login)
You should then list the other files it contains with
dpkg -L capnet-assist
In this case you'll see the capnet-assist does pretty much nothing else than providing the captive-login executable and some documentation for it. Knowing this you can then safely choose to remove the package capnet-assist, for example with
sudo apt-get remove capnet-assist
Now the package manager is aware of the change and will warn you if any other package depends on capnet-assist, in addition to not restoring the file next time capnet-assist is upgraded.
start.elementaryos.org: add an entry like127.0.0.99 start.elementaryos.orgto your/etc/hostsfile - then no process will easily access that page. You could, of course, also set your start/home page in your web browser and/or its system configuration file(s) and/or your desktop manager settings. – Ned64 Sep 08 '15 at 14:56