That's actually something that the other person's e-mail program is doing;
it's a very common thing, and I'm not sure of a way to get rid of them when
you get a message.
To get your own mails to send without these, do the following:
For HTML:
Go to Tools, Options
Go to the Send tab
Under "Mail Sending Format" click HTML Settings
Uncheck "Indent Message on reply"
Click Ok
For Plaintext:
Go to Tools, Options
Go to the Send tab
Under "Mail Sending Format" click Plaintext Settings
Uncheck "Indent Message on reply"
Click Ok
Now, if you'd like your own messages to look better, check out
http://home.cs.tum.edu/~jain/software/oe-quotefix/ OE-QuoteFix. I haven't
tried it personally, but it looks really cool, and while it won't solve all
your problems, it might help a little; it'll at least make it more readable.
Check it out, if it looks like something you'd be interested in. I think I
will likely install it at some point as it looks pretty neat.