Since you want to purge revisions on a field in a field collection and not an entire content type, your best bet is going to be doing this directly in phpMyAdmin. I don't know of any modules that enable you to delete revisions for a particular field but not the whole content type.
I would locate the field in question and run DELETE from field_revision_field_yourfieldname.
Of course you should back up things before you run a delete query.
The data for nodes and node revisions are store in two different tables. The data for the node page is stored in field_data_yourfieldname. The data for the revision is stored in field_revision_field_yourfieldname.
To illustrate this concept for you, I set up a fresh install of drupal (7.28) on my local machine and connected it to a new empty database.
I set up a content type called field collection demo and I added two field collections.
Since I'm not sure what your configuration entails, the fields I used may or may not be similar to your fields.
Then I created two sample nodes and edited both of them several times with revision tracking turned on. I mostly used lipsum text but for the last revision I typed meaningful text to help you see what's happening in the database.
Here you see that I have created the two nodes:
Here you can see the lipsum text in the original post.
Here you can see that I've created multiple revisions of the node.
And here you can see the meaningful revised text that is the most recent revision.
The next picture shows you how this data is stored in the database in the field_revision_field_yourfieldname table.
You can manually delete one row from the table using the red "X" in phpMyAdmin. Look, for instance, at the last row in this table.
After I click the red "X" for that row, it is deleted.
You can see that now the revision that began with r2 is gone from the field_revision_field_my_fifth_field table.
This removes the revision from the node's revision page. Notice in this picture, the field labeled "My fifth field" is missing.
But this does not affect the node's public view, because that data is stored in the field_data_my_fifth_field table not in the field_revision_field_my_fifth_field_table.
Therefore, I can safely delete all the revisions from the field_revision_field_my_fifth_field_table with the delete query (as I've done on my demo site) and still view the node with no problems:
All the revisions from the field "My fifth field" are gone on the revisions tab but the view tab remains unchanged.
If you wanted to delete all the revisions for all the fields that comprise a node collection, you would simply run the delete query for each of the field_revision_field_yourfieldname tables in the database.