Comment 17 for bug 393645

Aaron, in Karmic, the seahorse-plugins package is no longer installed by default so "Encrypt" and "Sign" are not shown in the menu.

You are right about using apt to remove seahorse-plugins, although users who would benefit from the removal of those entries are less likely to know the package name (and about apt, or using a terminal, or using sudo) than users who are familiar with those features and want them.