Hello Nelson, welcome here 
You can not mute given strings because MG does not recognise the different strings but the pitch of the notes you play: it considers i.e. that the note A at the 5th fret of the low E string is the same as the A open string.
You can however do some manipulations thanks to the scripts available in the Midi FX slots, for example:
-
“lowest note in bass range”: to play in the midi instrument only the notes lower than the selected one.
-
channel split" to split the range by selecting a given note, the lower notes will be played on a midi instrument in a midi channel and the higher ones on another midi instrument in a different midi channel.
There are many other useful scripts and you can also write your own.
Look at this video I recorded with a midi bass instrument:
lowest note in bass range