cowboy

noun (DISHONEST PERSON) 

/ˈkaʊ.bɔɪ/ [C] 

UK informal someone who is not honest, careful or skilful in their trade or business, or someone who ignores rules that most people obey and is therefore not considered to be responsible

Those builders are a bunch of cowboys - they made a terrible job of our extension.

You have tried the cowboys. Now try the Indians! (from an ad)

(Definition of cowboy noun (DISHONEST PERSON) from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)