Question: I am looking for a single word to describe a group of customers, who merely take advantage of the company's services (they come only for free period and leave afterwards or take the gift and leave again).

So far I came up with a term `Profiteers`, but this is somewhat pejorative, which is not what I want.

Answer: 
In the specific context of *credit cards*, the word “tart” (in the sense of prostitute) is used in combination with **rate** or **card** (at least in Britain). Thus, on [macmillandictionary.com](http://www.macmillandictionary.com/buzzword/entries/rate-tart.html):

> 
> **rate tart**
> 
> 
> noun [countable]
> 
> 
> 1. *also* **card tart** or **card surfer**   
> someone who continually switches credit card providers in order to get the lowest interest rates
> available
> 
> 
> 

Sorry, it’s not a single word, but the combinations represent specific usage. (And it’s not plural either, but you can add an ‘s’ if you like.)
[Transient(s)](https://en.oxforddictionaries.com/definition/transient) might fit your needs:

> 
> ADJECTIVE
> 
> 
> 1. Lasting only for a short time; impermanent.
> 
> 
> 1.1 Staying or working in a place for a short time only.
> 
> 
> 

and

> 
> NOUN
> 
> 
> 1. A person who is staying or working in a place for a short time only.
> 
> 
>