Competition will take some really drastic measures these days. The hard part is actually figuring out who did these heinous things. Recently I can across a posting about a man who had his email hijacked, had spam promotions sent from it in the form of promotions for a site he owned, and then had complaints filed against him by either the hijacker or the recipients. All this hectic activity took place in an effort to shut down the man's website. The worst part still is that the spam authority his email laws reported to, as well as his service provider, both thought he was guilty.
If you find yourself in this predicament, there are a few things you can do:
Start by looking up your email forgery and fraudulent identity laws for your state
Do an IP address search or contact the spam authority that was complained to and ask them for the mail's server. The system admin should be able to find it in their file. Find this and find where the email came from.
Once you find where it came from you can do a search to get the name and contact info of that person. Depending on the laws of your state, you may be able to pursue litigation.
Change your email address.
Get some firewalls, spyware protection, and antivirus protection.
From here you should be okay. Visit this Google Answers thread to read more on the issue and resolutions.