Blocking of users is one of the actions administrators can take to counter spam and other vandalism. Spam is clearly deliberate and needs no enquiry. Vandalism may, however, have been accidental and should usually be enquired into in case a legitimate user made a mistake, didn't realise it, could avoid it in future if told, and could be lost to us if unexplainably blocked. See the Block log to check whether such an offender has already been blocked. If two or more blocks have been applied, the one with first expiry date has effect. One month is a typical time for serious first offences.