Check the directional polar pattern of the mic. If the mic is the type that can be adjusted to different directional patterns, set it so it only picks up sound from the intended direction.
Place something behind the mic to absorb sound. Try using acoustic panels or some acoustic foam. Attach a special reflection filter to the mic stand if you have one. If not, you can improvise. Position a rolling clothes rack behind the mic and hang a blanket over the rack.
Fill the room's empty spaces with something to absorb sound. Use acoustic foam on the walls if budget permits. If it does not, use old quilts, mattresses, blankets or pillows.
Turn down the gain on the mic and adjust the source's proximity to the mic to achieve your desired source level.
Experiment with different mic positions. Sometimes a mic will sound better in an inverted position, or higher or lower relative to the sound source. Test it in several locations and see which one results in the least amount of echo.