You ought to be able to switch to the Text (HTML) editor and remove extraneous spaces. However, more important is understanding what's creating them. Are you copying and pasting text form somewhere else into posts?
If you have been copying and pasting text from elsewhere without using either icon 5 or icon 6 in Row 2 of the visual editor, then please see here http://en.support.wordpress.com/visual-editor/#pasting-text
Go to Settings > Writing and select “ ___ WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes.”
Posts > All Posts, change all your latest posts to Draft, click Update, change them back to Published, click Update again. (You can edit all of them at once: tick the square in front of their titles then select Edit from the Bulk Actions dropdown and click Apply.)