Having a yard alarm sign without actually paying for service?

If I post a yard sign from a real home alarm company that I purchased from ebay, without paying for the service, is there any way I can get into trouble with the company? And can they make me take it down?

How can I arm my alarm system with out having a monitor company monitor me?

I bought my house with a ADT system alredy instlled. I would like to arm the system when we go to bed at night but I do not know the code and the owners before us do not remember it. I was thinking I could go back to some original 9999 number and start form [...]

What questions should I ask an alarm company before having an alarm installed?

I am getting 4 estimates for an alarm installation and wondered what questions I should ask. Thanks for any suggestions.
This is for a home alarm