I would say without a doubt their banner design has been influenced by yours. The design is strikingly similar, so I do believe it is aimed at taking off yours. The header text is mildly suspicious;
<blockquote>
&n bsp; Dutch   ; TeenClub
Seventeens</blockquote>
I reckon having "...Club" and "Seventeens" in that kind of placement is indicative of a rip off, as well as the title, <i>Dutch Teen<b>club Seventeen</b>s</i>. Last but not least, the general theme - soft tones & girly graphics.
I'm not convinced the similarities are strong enough to hold up legally however. Alternately perhaps you could come up with a cunning plan?
If I was you I too would be pissed.
The million dollar question is, do you believe they're cutting into your bottom line? Also, do you forecast this to happen in the near future (how long have they been operating?)? If neither, and you can't take the legal route, I don't believe you should do a thing.