Thanks again!
I've decided to go with adding Parser::Number::NoDecimals(); and living with the possibility that some enterprising students might enter approximations as fractions.
I fiddled with using Context("LimitedNumeric"), but then WeBWorK complained about my using parentheses in expressions like "root(n,k)"
I didn't try to remove the "/" operator because I will also be writing exercises where the appropriate answer is "1/sqrt(x)"
Bruce