There are a few ways to make custom ISOs. For example: http://www.howtogeek.com/109736/
(should work with Mint, but not certain)
remastersys used to be the default for making custom ISOs, but the software has been discontinued. Someone else has continued the project; I haven't tried it myself, but you can check it out here: http://system-imaging.blogspot.com.au/
You may find it easier to install a smaller distro and convert it to Mint.
eg. If you wanted the XFCE version of Mint, you could install Xubuntu, add the Mint repos, update, and install the appropriate meta packages.
It can be more trouble than it's worth if you're after a version of Mint without an Ubuntu equivalent with the same desktop environment.