Cattails are tubers, just cutting them won't work, you have to dig the roots out of the mud.

It's a lot easier to spray them. Rodeo is the "approved" glyphosate for use around ponds but can be bought under the generic name "Aquastar" much cheaper.

http://www.cdms.net/LDat/ld5FH003.pdf

http://www.cygnetenterprises.com/Product.aspx?id=831&pid=0&mid=&cid=