there's no Mission in which you have to Spray
all the 100 tags!!!
if you're talking about this mission "
Tagging Up Turf", there's only 6- 8 tags you've to spray,Just spray over the face of COPs (OR kill him in a fight take his weapons) & just run away from their sight. the heat eventually cools down.
Even if the Heat level doesn't cool down, just try to finish the mission. You'll lose all the heat level.
[for loosing cops "TURNDOWNTHEHEAT" = Clear Wanted Level]
just go here as i've posted already ,you'll find whatever cheat you want.
http://thegtaplace.com/sanandreas/pccheats.php
If you're talking about SPRAYING ALL THE 100 SPRAY TAGS.. then this'll help you.
there're cheats for that, but they just pretend that you've sprayed all the tags.
Instead of lookin for cheats just use this "interactive map" to pin point the exact locations for all the Spray Tags.
http://www.gta-sanandreas.com/guides/tagsmap/map.htm