SpamShield is the term for any program used to protect against messages that contain unwanted information or memetic manipulation. The SpamShield can be set to several modes, depending on the region and the user's level of tolerance. The most common modes are 'off', 'reword' or 'delete'.
The 'off' setting is just that: everything comes through. It may be used in trusted or low-traffic areas, selectively with trusted sources, or carefully and over the short term as a way of sampling the social environment.
Rewording prepares a short precis of the communication that is engineered to be memetically null. An unsophisticated spamshield can produce very spotty results on this setting, particularly if it is dealing with a complex and highly sophisticated message or with a source that is subtler than the program.
On 'delete' setting a spamshield deletes anything which references unwanted subjects. As with rewording, the actual results depend on the relative sophistication of the spamshield.
In some regions spamshields are disallowed entirely or limited in scope on the grounds that they create solipsistic world-views that lead to social discord. In contrast, some polities that are ideologically based issue a standard spamshield as a way of ensuring that the local memeplex remains unchallenged.