Ornamental grasses rely on wind pollination so do not attract bees and other insects in the same way that flowering plants do. Not sure about spiders though - they seem to just go wherever they want (at least in my garden)

If you don't want bees, choose any flowering plant which has double flowers - these are sterile, so bees won't visit. As for spiders, well I can only echo Rogerbee's comments.

A nice japanese maple would surely look good, they do like a sheltered spot though, not sure about spiders though, they go everywhere dont they.

There is a spray you can get to repel spiders, if you copy and paste the address below into your browser there is a link. There is another one called cobweb eliminator spray, you will see it come up if you put it into google but I don't know uk suppliers for this one. Neither of these harm the spiders, but are supposed to repel them. There are also ultrasonic devices for spiders that claim to work but I don't know how well they perform.

http://www.amazon.co.uk/151-No-More-Spiders-Spider/dp/B002EISUUS/ref=sr_1_1?ie=UTF8&s=garden&qid=1274947454&sr=8-1

I would add that if you don't want bees avoid having a pond close to your decking as I have a pond and bees, hoverflies and particulalry wasps love it and come to it to drink. I would add I have never had a problem with them being aggressive towards me though and use the mints trick in autumn with wasps - a packet of mints left open keeps them happy and ensures their sugar needs are satisfied during this time and stops them from agitating around people.