Although it’s simple but it’s a little bit tricky. Adding / removing an event receiver to a list and content type is the same you just need to use the appropriate collection is an SPWeb object.

Here is how to add / remove an event receiver to / from a list:

# The following code registers an event receiver for when an item is being added to a list
$web = Get-SPWeb "http://yourwebsiteurl"
$list = $web.Lists["your list name"]
$list.EventReceivers.Add("ItemAdding", "Full Assembly Name", "Full.Class.Name")

Removing an event receiver is as easy as:

# Be careful about the index value for when you have more than one event receiver!
$list.EventReceiver[0].Delete()

As you can see below adding an event receiver to a content type is very similar:

# The following code registers an event receiver for when an item is being added to a list
$web = Get-SPWeb "http://yourwebsiteurl"
$ctype = $web.ContentTypes["your content type name"]
$ctype.EventReceivers.Add("ItemAdding", "Full Assembly Name", "Full.Class.Name")

And so is deleting it:

# Be careful about the index value for when you have more than one event receiver!
$ctype.EventReceiver[0].Delete()

Now more important is the tricky part. If you have used this content type to create other / list instances then you should also call the following command to propagate the change:

$ctype.Update($true)