Whether you use G Suite or Office 365 or an other connector, the approach is exactly the same.
To do so, go to Signitic, Settings -> Connector.
Click on the icon of the installed connector (O 365 in this example) to get to the page below.
To cut the synchronization, just click on Remove connection.
When it's done, signatures won't be pushed anymore and users won't be retrieve either.
Note : Before you get there, tell us why you wish to disconnect your directory: we will certainly be able to answer your questions !