fanerrrka

import re

for index, row in enumerate(df['cell_name']):
    corrected = re.match(r'^#(.+?)(?=#)', row)
    if corrected:
        df['cell_name'][index] = corrected.group()
#
# {'cell_name': ['#_marS_8_all_0M',
#   '#_marS_8_all_0M',
#   'Контрольная группа [ВС]',
#   '#_marS_8_all_0M',
#   '#_marS_8_all_0M']}

df = {'cell_name':
                ['#_marS_8_all_0M#_[BC]',
                '#_marS_8_all_0M#_[BC]',
                'Контрольная группа [ВС]',
                '#_marS_8_all_0M#_[BC]',
                '#_marS_8_all_0M#_[Баннер]']}

print(['#'.join(d.split('#')[:2]) for d in df['cell_name']])