this should work. you need to compare each letters one by one then append them to a new string
a1=[]
a2=[]
a1.append("abcdef")
x="abc"
y = "abcdef"
new = ""
for letter in y:
if letter not in x:
new = new + letter
a2.append(new)
print a1, a2
output:
['abcdef'] ['def']
[Finished in 0.0s]
this version will check each item in the list vs a single string to check if the string is in any of those
a1=[]
a2=[]
a1.append("abcdef")
x="abc"
new = ""
for item in a1:
for letter in item:
if letter not in x:
new = new + letter
a2.append(new)
print a1, a2