library(mice)
library(missForest)

set.seed(123)

# Create a dataset with missing values on 2 columns
iris.mis = prodNA(iris[, c('Sepal.Length', 'Petal.Length')], 0.20)
iris.mis = cbind(iris.mis, iris[, c('Sepal.Width', 'Petal.Width', 'Species')])

# Setup MICE
init = mice(iris.mis, maxit = 0)
iris.metod = init$method
iris.method['Sepal.Length'] = "" # Do not impute Sepal.Length

# Run MICE
imp = mice(iris.mis, m = 5, maxit = 5, method = iris.method)

# Inspect Result
res = mice::complete(imp, 1)
print(length(res$Petal.Length[is.na(res$Petal.Length)]))

# [1] 8
# i.e. still 8 missing values in Petal.Length, when we wanted 0