1) Read enough of t hread before commenting. Many people will quickly glance through a thread and write a reply that has little contribution to with the topic on hand. If you get no replies, your post is probably meaningless.
2) Be controversial. Say you opinion and speak your mind about the topic at hand. This will result in possibly heated debates that can get you really enticed in the conversation. You may even forget you're being paid to post on this forum.
3) Write write write. As a general rule of thumb, new threads should be a few paragraphs long and replies should have at least four sentences. Short replies posted just to be paid is spamming.
There are my suggestions. I followed these rules and haven't been banned for spamming or removed from my campaign. If you're one of those people who don't give a shit, you may end up banned.
I have a problem with number 2.
If you try to forget you are posting for signature (as I used to do). Once you have a rational conversation with a person, you are accused of spamming by another member.
Possibly these will reduce nonsense spamming anyway of 1 letter/word or even sentence replies.
I know that I personally try to make my posts a few senteces or lines per post each.