library(party)
model_cforest <- cforest(drat~.,data=mtcars,controls=cforest_unbiased())
cforest_var <- varImp(model_cforest,conditional=TRUE)

Overall
mpg   0.009778909
cyl   0.033507134
disp  0.056359569
hp    0.000000000
wt    0.044186730
qsec  0.000000000
vs   -0.000309504
am    0.050791540
gear  0.060967894
carb  0.000000000

removeNeg <- function(data){
  model_cforest <- cforest(drat~., mtcars,controls=cforest_unbiased())
  cforest_var <- varImp(model_cforest,conditional=TRUE)
  varImp_neg <- row.names(cforest_var)[apply(cforest_var, 1, function(u) any(u < 0))]
}