Add this line in your EditText tag in layout(xml) file or you can read more about this here
android:digits="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
Full code EditText should be look like
<EditText
android:id="@+id/edittext"
android:digits="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
android:layout_width="fill_parent"
android:hint="Enter Full Name"
android:layout_height="wrap_content" />