print (df['S'] != df['T'])
0    False
1     True
2     True
3     True
4    False
5     True
6     True
7     True
8    False
dtype: bool

df = df[df['S'] != df['T']]
print (df)
   S  T  W           U
1  A  B  0  Undirected
2  A  C  1  Undirected
3  B  A  0  Undirected
5  B  C  1  Undirected
6  C  A  1  Undirected
7  C  B  1  Undirected