Firstly, realise that you do not connect to 'mIRC', you connect to IRC. m
IRC is called a 'client' - the client allows you to connect to IRC. There are many different clients out there that will let you connect to IRC, mIRC is the most popular one.
Secondly, have you registered your nickname with the network? Some networks do not have nickname services (most of the big ones don't) and so if someone is using a nickname that you use regularly that doesn't mean they are using YOUR nickname, it means they are using a nickname that you use regularly
- If you have registered your nickname with a nickname service however (such as NickServ) then there is often a 'Ghost' command that you can use to kill off the person using your nick.
For help with Services, go to the #Help channel on your network and ask them for some advice and help. Services have nothing to do with mIRC - they are unique to the network that they are on. As I said, some networks don't have Services at all, and different networks have different Services...You may wish to check out the network's website for relevant documents.
Hope this helps