Add negative keywords like: -motorbikes This will exclude the unwanted keywords. or set the brackets: [buy motorbikes london] to ensure that only exact matches will cause impressions.
Negative is a good idea, howeer they will be many and odd words. The quotes as I understand wont display buy blue cars in london if I have keyword buy cars london. As far as I can read in google support the best should be to use the + for example like this buy +cars +London as cars and london is a must in the phrase, I will wait some days to see if it will be better as there always are bad days. Thanks