Blocking a forum with robots.txt

[lateuk]

Hello,
I am moving from one forum software to another, i currently have about 2400 forum posts listed on Google that will all 404 once i change to the new forum. Should i block the forum with robots.txt now so they get removed from Google or should i just accept i will have a lot of broken links on Google for a while?

Also will this affect the rankings of the rest of my site in Google?

Thanks

[manish.chauhan]

You should block them from robots.txt and then remover those urls from google index using google webmaster tools..:)