This will be dependent on the network.
If you were infected with a trojan, then you're probably banned from the network for a period of time (which you should've been informed of) or you need to use the networks website to let them know that you have removed the trojan and that you would like back on the network again. (You should've been informed of this also, if this is the case).
My suggestion, since you haven't indicated which (if either) of the above responses you received, is to use the network's website, as attempting to connect to the network while banned can trigger a permanent ban for ban evasion.