Thank you folks.
Could somebody please let me know about what one can do to hurt my domain(s) that they were working for, and, let's say, by some reasons they don't anymore.
They will not have an access to the servers, but I think if someone wants to do a damage they can submit it to search engines a way that it will be sandboxed, right?
What else they can do?
I have a feeling that after exposing some of my sites at some webmasters message boards the alexa for them nose dived... How did they do that?
@kesha1 - just pick something you want to try , and then give it a try.
Backlinks from social bookmarking sites for example:
Go the the services forum and look for a thread by someone selling that service. read through the thread to see what kind of reviews the guy has. Look at the post date from the start of the thread, and if it is still recieving new replies. If it's a really long thread, read through the first page or so then skip the the last page and read the most recent reviews. Go through the posts backwards till you find the most current price for the service and then give it a try.
Then pick another service:
Try directory submissions, or getting a linkwheel built, or something else you've read about that seems like it might help your site. Do the same thing again to look through the threads and read previous reviews, and then give it a try.
Always choose a small package to start so that you aren't out much money if it turns out to not be what you were expecting. Leave your feedback based on whether the service was performed, not on how well it worked for you. You can also include in your review what you were hoping for and that you think it wasn't the right service for that type of project. But don't give a bad itrader or neg rep on the service provider, you just now know what that service entails.
Try the safer services first like social bookmarking etc. Once you see what results you get from those type of services, then move on to the ones that have more risk of problems like linkwheels and xrummer services.
If you follow that advice it is unlikely that you'll have any of the problems you're worried about.
You would have to really piss someone off to get them to waste their time intentionally trying to damage your website. Not saying it could never happen, but if someone has been providing a service for awhile, and other people are satisfied with the service, then it's very likely the service provider knows how to be professional.