Preventing a Comma in text fields

Hi,

I'm trying to create a validation rule to prevent special characters,
I'm able to add almost all special characters and it works fine..... except the Comma ( , ),
You use the comma to separate between the values in the condition, but what about the comma itself?
How can you prevent it? I tried entering: ,, but the system ignores it.
If I enter just: , the system converts in to 'null'....

See attachments below.

Any help?

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