There are two ways to exclude objects from being synchronized.

On the Export Side:

To exclude object from being exported at the source, please add the value NOGALSYNC (not case sensitive) to any extensionAttribute (1 through 15).  The next time the export policy runs, objects that contain this value will not be exported.  The following cmdlet can be used to quickly add this value to an object:

Set-ADUser -Identity "Username" -Add @{extensionAttribute1="nogalsync"}

On the Import Side:

To filter objects from being imported to the target, please select the import policy node > directory tab > Settings > Object Filter then select the property and condition in which to filter objects: