Removal of unwanted frequency components

I am presently using OrCAD to design a communication system. After successful design of Transmitter and receiver, I am encountering a problem regarding unwanted frequency components present in the recovered signal.

I sent a sine wave with frequency: 500 Hz. After reception and reconstruction, this is what FFT of the signal lloks like:

**broken link removed**

Is there any way of getting rid of the unwanted frequency components (other than the peak at 500 Hz)?

There are some points to mention:
1) The entire system is baseband transmission
2) The message frequency can lie anywhere between 100 Hz to 5 kHz and can also be a summation of sine waves having frequencies in the given band.
3) Noise too lies exactly in the same band.
4) The message frequency component will always have greater amplitude than other frequency components.

You'll preferably use a message coding method that isn't sensitive to smaller interferences.

Most communication standards are desgned that way. On the other hand, you may want to find out how the interferences are generated.