Create a script to perform a TextFormatRemove (), then call that script using the script trigger: onModify for the field.
Create a script to perform a TextFormatRemove () … using the script trigger
An auto-enter calculation will do that a bit more elegantly …
I hope your day is going well. Another way you might consider achieving your goal, which is very similar to the method Jamiebah suggested, is to place the TextFormatRemove (Self) calculation in the AutoEnter Calculation option for the field on the table and ensure the "Do not replace existing value..." is NOT selected. This way would make the format removal independent of the layout, if that is what you need.