from imaplib import IMAP4_SSL

HOST = 'mail.XXXX.com'
USERNAME = 'XXXX'
PASSWORD = 'XXXX'
SUBJECT_CHECK = b'[repository/thing]'

with IMAP4_SSL(HOST) as m:
    m.login(USERNAME, PASSWORD)
    m.select('INBOX')
    result, ids_raw = m.search(None, 'ALL')
    assert result == 'OK'
    ids = ids_raw[0].split(b' ')
    counter = 0
    for m_id in ids:
        if counter > 100:
            m.expunge()
            break
        subject = m.fetch(m_id, '(BODY[HEADER.FIELDS (SUBJECT)])')[1][0][1]
        if SUBJECT_CHECK in subject:
            m.store(m_id, '+FLAGS', r'(\Deleted)')
            counter += 1