block ? with robots.txt or htaccess

[zeus]

How can I block mysite. ​com​/?i​d=2​891​&ca​t=bla I get a lot of duplicated content of my frontpage, they all starts with ? after the domain

is this the right way in robots txt

User-agent: *
Disallow: /*?

I dont think so, be cause it blocks ? any where right, I just want if it comes after the domain

[Hoegaarden]

You really should be looking into the creation of query string URLs rather than blocking from search engines. Why is your website creating these pages in the first place?
Anyway if you do not want to answer for any reason, in your Goolge Webmaster Tools account you can tell Google Not to index these URL Parameters ?whatever

[g1smd]

Parameters right after the domain would be:

Disallow: /?

You should also add

Disallow: /index.php?

or similar.