Besides spam, the other big no-no is to try to manipulate voting by any means: manual, mechanical, or otherwise. We're not going to post an exhaustive list of forbidden tactics (lest we give people ideas), but some major ones are:
Don't use shill or multiple accounts, voting services, or any other software to increase votes for submissions
Don't ask other users to vote on certain posts, either on reddit itself or anywhere else (through Twitter, Facebook, IM programs, IRC, etc.)
Don't be part of a "voting clique" or "vote ring"
You should not ask for votes on reddit, even on your twitter or blog or forum - it will get your account banned, and in extreme cases can get your domain banned
A voting clique is a group of people who send links to their submissions around via message, IM, or any other means, with the expectation of "you guys vote for my stuff and I'll vote for yours." A "vote ring" is a group of people who agree to vote on certain things together, either a specific submission, a user, a domain, or anything like that. Upvote each submission or content for the value of the information in it, a variety of things that you think are interesting and will benefit the community.
You may never offer money or compensation to anyone to promote anything on reddit for you. Things should be submitted on reddit by redditors who have found your content organically and submitted it because they found it interesting. You should never run a contest to see "who can get a link highest on reddit" or ask anyone to submit links on your behalf.
You should not spam in any way, especially through private message. You should not hide your affiliation to your project or site, or lie about who you are or why you like something. You should never, ever buy votes or ask for votes - if your stuff is good enough, people will vote for it naturally. If all of your employees or contributors vote on all of your links, their accounts and possibly your domain will be banned.
Don't pull some sort of sockpuppet viral marketing shit on reddit. It's tacky and cheap, and detracts from your brand. Additionally, it can get your domain or brand banned from reddit, and these things have a tendency to go viral and create a backlash against you. Be authentic and honest and things will go much better for you.
I made the damn subreddit I was promoting. The thing had nothing to do with my website so what justifies their action to ban a website I own?
They're thinking just like government that having a rule to ban or regulate something will stop what they don't want.
NO. It shuts out anyone caught; and the big players can usually find the best way to manipulate unseen. They're the winners.