Each logging message has a default severity level associated with it. You can change that default behavior so that a message is sent based on a configurable severity level instead. For the messages that have a higher default level and that will not be sent, you can reconfigure their level to a lower value.
To change a message's severity level, use the following configuration command:
My understanding is that you see the messages related to 'ftp' in the ASA local log. If so, one way I can recomend (there may be different way, but Iam not sure..;-)) - using the Message list. For this first find the message ids for the ftp related connections from ASA logs then create message list based on that. ex:
logging list my_CRITICAL level warnings
logging list my_CRITICAL message 111001-111009 logging list my_CRITICAL message 611103
logging trap my_CRITICAL
This will send 'warning' and any log messages Ids matches between 111001-111009 & 611103 as well.