Using an if else for a value in one table to insert data into another table

I want to allows a user to add data to my 'posts' table if the user entry in my 'users' table has a banned_till value of either nil or less than current time.

Something like (pseudocode);

If (select banned_till from users where userid = $1) = nil or < currentTime
   Insert in posts (col1, col2) values ('abc', 'xyz')
ELSE
   RAISE Exception "User is banned"
ENDIF

Currently i am doing this using 2 queries; first one checks if the user is banned and then the 2nd one inserts into the posts table. I'd really want to combine them into a single query, if i can.

Note: I'd really prefer not using and stored procedures or something too specific to a SQL DB. Something simple and universal is much preferred.