The forums went down, so the old links won't work anymore, however I saved most (or even all) of them and you can download them here:
http://www.me diafire.com/file/7nnlcvmi67bga28/Blockstorm+Prefabs.zip (remove space)

This collection also includes my own prefabs and prefabs I made from other people's maps.
The prefabs are divided in multiple theme folders, there might be a few mistakes though.

You can also find some prefab collections on the Steam Workshop:
http://psteamcommunity.yuanyoumao.com/workshop/browse/?appid=263060&searchtext=prefabs&childpublishedfileid=0&browsesort=toprated&section=readytouseitems

I hope these prefabs will help you building your maps!

Actual 'How to make Prefabs' Guide:
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=697345557

This guide isn't going to show you what prefabs are and how to make or use them,
I'll just show you where you can download or post prefabs.

Prefabs page on Blockstorm forums:
http://forums.playblockstorm.com/index.php?forums/prefabs.20/

Several prefab themes:
http://forums.playblockstorm.com/index.php?threads/halloween-horror-theme.354/ (Horror)
http://forums.playblockstorm.com/index.php?threads/star-wars-theme.367/ (Star Wars)
http://forums.playblockstorm.com/index.php?threads/medieval-theme.379/ (Medieval)
http://forums.playblockstorm.com/index.php?threads/world-war-ii-theme.392/ (World War II)
http://forums.playblockstorm.com/index.php?threads/city-theme.584/ (City)

Place the downloaded prefab files in: C:\...\AppData\LocalLow\IndieGala\Blockstorm