Custom Error Message Based On Conditions..?

[C Braithwaite]

Hiya all - i was wondering how i could display a custom error message based on some conditions..

for example - keep redirecting to custom error message untill a user votes on a poll..

i have all my code wrote, and my variables ready.. i just need to force my members to do something (which will be displayed on the error page) before they can browse the rest of my site.