To answer your question specifically:
If the redirect that they have setup is infact a 301 redirect, then there is actually no danger whatsoever because that tells a search engine that the .com version is the real deal. If they setup an unusual redirect, like a http meta refresh, for instance, with some text on their website that might be trying to hijack your search engine ranking, then, yes, you may have something to worry about as that domain could come to compete with your .com version over time.
The process for dealing with this is only a legal one as the other party may be violating your trademark (registered or implied). A registered trademark always helps your case. I've been in a similar situation before but there are two things to keep in mind here:
You must be able to prove that you have had the domain and business registered before the other party registered its domain or its business online with that domain.
They really must be posing an actual risk to your business or website (in terms of taking away users, search engine rankings or potentially lost business, etc.), otherwise your case is weak, particularly if the trademark is not registered.
Finally, of course, you'll need to get an attorney / solicitor involved. From my past experience, this is, unfortunately, a little about intimidation along with the fact that your demand that they cease to use your domains is a valid one.
A letter from a lawyer might mean they will stop using the domain and de register it or transfer it to you. If it's an international party that won't care about what your lawyer or your court jurisdiction says, the court can order the registrar to cancel the registration or transfer it to you, but for this you really need to have your intellectual property in place.