Having this Spam attack:

warning access denied 2006-08-10 04:16 user/1 denied access. Anonymous view
warning page not found 2006-08-10 04:04 Abstract not found. Anonymous
warning access denied 2006-08-10 03:47 disknode/browse denied access. Anonymous view
warning page not found 2006-08-10 02:49 _vti_bin/_vti_aut/author.dll not found. Anonymous
free texas hold em active 6 hours 59 min never edit
7 card stud active 8 hours 51 min never edit
party poker com active 8 hours 52 min never edit
online poker game active 9 hours 1 sec never edit

How can we react to Stop?
Any barrier know?
Captcha doesn't work in our 4.7.2 (it has a bug no appearing in Access Control).
Any Good Practice documented?