Here is a list of reported bad bots
its up to you as a site owner to decide to allow a bot or not as everyones reasoning can be different however some bots you just should not allow ever.
Bad bots as they are called can be a simple thing like just using up bandwidth with no purpose,content scrapers,ignoring robots txt,comment spammers,email harvesters and more
these are ones i always ban
ac-baida
AnotherBot
botpaidtoclick
Click Bot
cr4nk
curl
DA 5.3
DataCha0s
discobot
EBM-APPLE
EmailSearch
EmailSiphon
FAST ESP Document Retriever
Firefox 2.0
Ginxbot
GrubNG
gvfs
HTTrack
Incutio
Indy Library
Internet Ninja
Java
JetBrains
libcurl
libwww-perl
lwp-request
lwp-trivial
Macintosh; I; PPC
Microsoft Data Access
MJ12bot
Morfeus Fucking Scanner
Mozilla Firefox 5.0
Mozilla/4.0(compatible
Mozilla/4.08
Mozilla/4.61 (Macintosh
Mozilla/5/0(compatible
Mozilla/7.0
Mozilla/8
Mozilla/Firefox
Mp3Bot
MSIE6
NIPGCrawler
paperlibot
PEAR
PECL
PHPot
Provider Protocol Discover
PuxaRapido
PycURL
Python-urllib
Security Kol
Site Sniper
SkyGrid
Sogou
sun4m
Sunrise
syncrisis
topicblogs
User-Agent
w:PACBHO60
W3CRobot
Web::Scraper
WebDav
WebRipper
Wget
window.location
Winnie Poh
Ranks Webmaster Tools
X12R1
Xerka-bot
please if you have any bots you know are bad then add to the list and if you only have a few to report maybe add a reason why you ban them