Hello, you can use the pyVCF python module to do this. Something like this should do the trick (the function deleteGT works but I didn't tested the rest of the code) :

import vcf
from random import randint

def deleteGT(record,VCFReader, sample_name):

    """ deleteGT(record,VCFReader, sample_name) ----> ModifiedRecord
    VCFReader: A vcf reader object from vcf module
    record: a record contained by VCFReeader
    sample_name: name of a sample contained in VCFReader
    ModifiedRecord: record without the genotype associed to sample_name """

    samp_fmt=record.FORMAT
    fields=samp_fmt.split(':')
    new_gt = './.'
    new_CallData = namedtuple('CallData', fields)
    calldata = [new_gt] + [None]*(len(fields)-1)
    record.samples[(VCFReader.samples).index(sample_name)].data = new_CallData(*calldata)
    return(record)

VCF=vcf.Reader("path_to_your_vcf")
OUT=vcf.Writer("out_path", VCF)
Samples=VCF.samples
N_samps=len(Samples)
for record in VCF:
       if (randint(0,100)>95):
            samp_to_del=Samples[randint(0,N_samps)]
            record=deleteGT(record,VCF,samp_to_del)
      OUT.write_record(record)