The Alerts window dialogue is identical to the filters window.

Adding entities to your alerts will cause it to blink on the map and appear in bold on the spawn list.

Filters

Adding entities to your filters list will cause it to not appear in the spawn list and if you untick "show filtered" (right click map window) then it won't be shown on the map either.

Filter On - When filtering out spawns you have the choice of matching the filter substring with either Name, Level, Class, Race, Info, HP, X, Y, Z or ID.
Regular Expression - This is the substring that Sins matches with the spawns to decide which ones to filer out.
Active - You can deactivate filters and leave them in your list, this makes it easy to toggle some filters on and off without having to retype them in and delete them.