Most of the time, you should be storing plain text and any formatting is applied at the presentation (aka layout) level.
If you actually want to store formatting text, ou can define an auto-enter calculation that applies text formatting functions to Self. See FileMaker's help or a calculation box fucntion drop down for a list of text formatting functions.
You can use this calculation in a replace fields operation to strip the formatting out of existing records or records that were just imported. You can also use it as an auto-enter calculation to strip out the formatting during import if you enable auto-enter options during import:
TextFormatRemove ( Table::FieldName ) //in replace field contents
TextFormatRemove ( Self ) // as an auto-enter calculation.
Also clear the "Do not replace existing value..." option when setting this up as an auto-enter calculation.