I keep getting notifications from a sweepstakes website on my iMac.

I can’t figure out where they come from, or how to get rid of them.

What can I do?

You have somehow turned on “notifications” in either your Yahoo Mail or the Mac operating system.

Yahoo Mail notifications tell you when a new email arrives in your inbox, including a spam email. Mac notifications come via your web browser or apps installed on your computer.

There are a couple of ways that Yahoo Mail notifications reach you:

• If you view Yahoo Mail through the Safari, Chrome or Firefox browsers, you will only get notifications when you have the Yahoo Mail website open. You can use the Yahoo Mail website to turn off those notifications (see tinyurl.com/y3h8f8ff).

• If you view email via the Yahoo mail app, you will get notifications all the time, regardless of browser activity. On a Mac or PC, you can turn off notifications in the mail app (see tinyurl.com/y6pvm75h — turn notifications off instead of on). Readers who use the Yahoo Mail app on a phone or tablet using the Google Android or Apple iOS operating systems should turn off notifications via the device’s settings (see the same website).

On a Mac, the operating system allows you to receive notifications via apps to which you have given messaging permission. To turn off these notifications, change the settings in the Mac operating system’s Notification Center (see tinyurl.com/ltlfnr6).

I keep getting pop-up ads on my tablet computer, a Samsung Galaxy Tab S2 that uses the Android operating system.

How can I block them?

The pop-ups could come from your Google Chrome browser or a malicious app.

Try turning on Chrome’s pop-up blocker (see tinyurl.com/y3we7lfl). If that doesn’t stop the pop-ups, they are probably caused by an app on the tablet.

You can try to find and uninstall the app by using ad detection software, such as AppBrain Ad Detector or Addons Detector (both are in the Google Play store).

Or you can try to find the app yourself: When a pop-up ad appears, press the “Recent Apps” button, which is to the left of the “Home” button at the bottom of the device. (Pressing the button once shows which apps were most recently used; pressing and holding the button shows which apps are currently active.)

Look at the resulting lists quickly, because some malicious apps can disappear from a list in seconds. When you know which app it is, uninstall it.

If neither approach works, you can return the tablet to its factory settings (see tinyurl.com/y2el5u8y). You will first need to back up your “media and pictures” data, then restore it after the reset (see tinyurl.com/yxneg96d). Don’t back up your apps — you would just be backing up the malicious app, too. After the reset, reinstall the apps one at a time to identify the one containing malware.

Or you can leave the malicious app in place and combat it with an ad blocker program (see tinyurl.com/y4ntv2z6).

Steve Alexander covers technology for the Minneapolis Star Tribune. Readers can send their questions to him at steve.j.alexander@gmail.com. Please include a full name, city and phone number.