The best knot to use for tying spectra is the bowline.
http://www.hightrek.co.uk/climbing/how2/bowline.htm
No matter how tight the rope becomes, you'll be able to remove the knot later. You need a couple of sailors on your team
Also note that if it is high quality spectra the end will not melt well. This is usually how you would keep a polypropolene or nylon line from fraying further after it is cut. To keep it from unraveling, get a bit of shrink wrap and shrink wrap the end of the line.