A team of Conservative Party researchers have been wasting time and money trying to dig up dirt on UKIP candidates rather than trying to win elections. They have compiled dossiers on UKIP candidates from their Facebook and Twitter activities, even criticising someone's dancing ability in a Facebook video and suggesting that a picture of an undergraduate in a black tie undermined his bid to become a local politician.
As Nigel Farage told the press, UKIP doesn't have the time or resources to vet every one of its 1,700 candidates centrally and relies on branches to vet their own council candidates (Westminster and EU candidates are vetted centrally). People joining the party sign a declaration that they aren't past or present members of a number of parties and organisations including the BNP, National Front and English Defence League.
In most cases the nutters and undesirables are either prevented from joining or caught out soon afterwards and expelled from the party but sometimes they aren't. It should be remembered, though, that UKIP is the only party that bans these people from being members whereas the LibLabCon and others have no problem with former Nazi's standing for them or even actively recruit members of racist groups.