You should use the linewidth
argument, which has to be set to zero.
Example (based on your other question's answer):
import matplotlib.pyplot as plt
import matplotlib.patches as patches
import numpy as np
fig = plt.figure()
ax = fig.add_subplot(111)
# generate some data:
x,y = np.meshgrid(np.linspace(0,1),np.linspace(0,1))
z = np.ma.masked_array(x**2-y**2,mask=y>-x+1)
# plot your masked array
ax.contourf(z)
# plot a patch
p = patches.Rectangle((20,20), 20, 20, linewidth=0, fill=None, hatch='///')
ax.add_patch(p)
plt.show()
You'll get this image: