If they are using your trademark, or deliberately passing themselves off as being related to your business, you have a strong basis for using a dispute resolution procedure or taking court action against them.
If they can claim that they are using the domain name legitimately, it may be much more difficult to take action against them. Rather than spending time and money on a dispute, you might be better off focusing on promoting your own domain name. An experienced advisor can help you decide whether you have a strong case and what the most appropriate course of action is.
Separately, there can be cases where another domain name is being used to take advantage of your customers. An obvious example is 'phishing', where phoney emails and a rogue website are used to collect information such as credit card details or security codes. While you are not responsible for this, you may want to warn your customers if you become aware that this is occurring. You may also want to establish an appropriate security policy and tell your customers about it: for example, stating that you will not ask for financial or security details by email.
