What's on the other side? Is there a window well?

Instead of removing the window, prevent the chance of water entering.

Val,

To eliminate the window and prevent future flooding, you will need to:

1. Remove the old window and frame.
2. Waterproof the exterior by filling the opening with concrete or masonry and applying a waterproof membrane.
3. Insulate and finish the interior to match your existing wall.

Important: Due to the risk of structural damage and potential building code violations (especially related to egress windows), it is highly recommended that you hire a professional contractor for this job.