To Stop Sharing a Single Entry
Locate the entry in your password list and click on it to open it. from within the Entry window, click the sharing tab to the right. You will see a list of people. Those people that you are currently sharing the entry with will have a check next to their name:
To stop sharing this password with a person, simply uncheck the checkbox next to thier name, then press the OK button in the entry window to apply the changes.
Do this separately for each of the entires that you'd like to stop sharing with a person.
TIP: To quickly find all the entries you are sharing with a given person, you can use the Quick Filters in your password list. In your password tabs, look to the left of youer password list. Near the bottom right of the Quick Filters box, click the show more link. Click the plus sign next to the You shared item. This will show you a list of names of people you are actively sharing with. Click on the person's name you are intersted in. Your password list will now only show those entries you are sharing with that particular person.
Immediately Stop Sharing ALL Entries with Someone
Under your People tab, locate the person's name. Click on the Stop Sharing link. This will immediately break all sharing you have with this person.
For Example, in the image above Tara Kelly would no longer see any of your passwords that you had shared with her. But you would also stop seeing anything that tara had previously shared with you. It's a two-way street.
Should you choose, in the future you could start sharing with Tara again.