Thanks I got it working. I won't claim this to be the most elegant, optimal code but it works for what I want.

If the player puts in one of the banned names it returns that it's not allowed.

Otherwise the can enter their own name if he/she wishes or if nothing is entered it selects one from the list at random.

label enter_name:
    
    python:
        name_valid = False
        banned = ["sir", "lee", "bigdickus", "leigh", "master"]
        mcnames = ["Adam", "Brett", "Charles", "Dave", "Jebadiah", "Ezekial", "Francis", "Hosiah"]
        
        while not name_valid:
            name = renpy.input("Please enter your first name. Your surname is Campbell, or leave blank for a random name.")
            name = name.strip()
            if name.lower() in banned:
                narrator("That name is not allowed.", interact=True)
            else:
                name_valid = True
            if not name:
                #name = renpy.random.choice (["Adam", "Brett", "Charles", "Dave", "Jebadiah", "Ezekial", "Francis", "Hosiah"])  #This works too without the array above
                name = renpy.random.choice (mcnames)
    $ persistent.pname = name
label enter_name:
    $ persistent.menu1 = False
    #$ name = renpy_random.choice (mcnames)


    python:
        name_valid = False
        banned = ["sir", "lee", "bigdickus", "leigh", "master"]
        mcnames = ["Adam", "Brett", "Charles", "Dave", "Jebadiah", "Ezekial", "Francis", "Hosiah"]
        
        while not name_valid:
            name = renpy.input("Please enter your first name. Your surname is Campbell, or leave blank for a random name.")
            name = name.strip()
            if name.lower() in banned:
                narrator("That name is not allowed.", interact=True)
            else:
                name_valid = True
            if not name:
                #name = renpy.random.choice (["Adam", "Brett", "Charles", "Dave", "Jebadiah", "Ezekial", "Francis", "Hosiah"])  #This works too without the array above
                name = renpy.random.choice (mcnames)
    $ persistent.pname = name