Hi Tony,
At some point al FWB will ask for exclusivity if she becomes too attached, if she doesn't become attached she will usually drift to another guy and you'll get ghosted. All we can do is prolong the time until the ghosting comes.
Try to express your disinterest in a relationship with your body language.
1. Plan your dates for the same night and not too far ahead - if you plan too much you come across as reliable and stable
2. Don't talk about building a future with her e.g. don't talk about mortgages, buying a house, holidays etc
3. Don't bring up your family or give her insight into your family dynamics
4. Avoid introducing her to your friends and decline invites to meet her friends
5. If the subject of marriage or kids comes up say you don't want either
If she asks you explicitly for exclusivity, then I would either avoid the question or be honest but tactfully. Something like "I don't want the responsibility of having a girlfriend at this stage of my life". Don't try and justify your answer and stay firm.
Basically you are creating red flags, if she stick around she will know its only casual...but as I said before she will eventually ghost you for someone who will commit.