I have so many people asking how in my Discord PMs, so I'm writing this so I could copy-paste the link...
This will be a step-by-step guide. But please use the Forum's search bar or Google search in case you are not familiar with some terms or programs.
Client Part [2015-10-01aRagexeRE to 2017-06-14aRagexeRE]
1. There's a patch in NEMO that can disable Summoner, use it.
Client Part [2017-06-21RagexeRE or later]
1. Use GRF Editor to open your GRF, search for ExternalSettings