Its best to use Revise.jl for this. Put your function in a file then run using Revise; Revise.includet("thatfile.jl"), or put it in a package in which case just using Revise, YourPackage. Then edit the source file and Revise will live-update changes to it into your running session, including deleting a method if you delete it / comment it inside the source.
In a pinch you can technically also do this by hand using Base.delete_method but its not nearly as user friendly.