So as i see it learning to play this game the overlord can essentially keep you in one section of the dungeon permanently. As the hero player i can never leave that area since he always is generating monsters and the resources to create them each turn. I cannot possibly cover the LOS of every room to stop them from spawning. This is preventing me from opening new doors and exploring further as the overlord can just keep popping down a horde behind me that i have to keep dealing with. Is there something i am missing in the rules or is there some trick to stop the overlord from spawning so many monsters?

Several things:

• You don't have to kill all monsters. If the monsters are far enough behind you that they aren't going to catch up and kill you right away, then just ignore them until they become a threat.
• Make sure you're using effective weapons (generally something with a green die). If you're playing with 3-4 heroes, you should generally be able to kill spawned monsters and do something else on the same turn (most spawns should die in about 1 attack, and each hero can either make two attacks or make one attack and do something else). If you have to backtrack a significant distance to attack the monsters, you probably shouldn't bother, as noted above.
• If you're playing with less than 3 heroes, don't ; the game's scaling mechanisms are really inadequate, and the fact that 2-hero parties are especially vulnerable to spawns is one of the major reasons why.
• The overlord only gets 2 cards per turn, and only 1 in 4 cards (in the base game) is a spawn card, so he shouldn't be able to spawn every turn, even if he has infinite threat and plays every spawn card he draws. And a third of those spawn cards only give him 2 monsters, instead of 3. He may have the resources to spawn for several turns in a row, through.
• Make sure that you're limiting the overlord to 1 spawn card per turn.
• Heroes can also help mitigate the effects of spawns by making strategic use of vitality potions and Guard orders.

If you still can't seem to make progress, then there are house rules you can try:

• You can use something like the reinforcement marker from the Road to Legend expansion; basically, if the overlord wants to play more than 1 spawn card per area, he has to pay extra threat (15, in RtL) for each one after the first.
• You can just make spawn cards more expensive, or let them create fewer monsters.
• You can remove some spawn cards from the overlord's deck.
• You can try playing The Enduring Evil , my personal attempt to rebalance Descent.

Just to add, keep in mind that Descent is not about "clearing" a dungeon, it's about accomplishing a goal (as defined in the different Quests). So if the OL spawns something behind you, so what? Usually you'll have good enough LOS that the spawn is /way/ behind you. Just ignore that creature and keep pushing forward.

-shnar