Marketo does not restrict or validate values for fields. Field values can be controlled by the type of field you use in your form. For example: Using a dropdown/select/picklist will allow you to control the values entered into Marketo. If you import a lot of lists it is best practice to have a process to normalize picklist values before you import the list into Marketo. If desired you can also use Marketo to correct, normalize or group values. It important to understand the implications and timing of normalizing values using Marketo if a new record will immediately inserted into your CRM.
If you want to clear a value from a field use the Change Data Value flow step and set the new value of the field to NULL.
Here are some resources for learning about the Marketo integration with SFDC.
.png)