As Tinkster says, if you want to change it permanently, you need to track down the line where it's being set, and modify that.
But to remove an entry from the current PATH "on the fly", you can use a simple parameter substitution
Just take care to note which side the colon needs to be on, if the pattern is at the beginning or end of the path.
I use this technique in a small function that toggles the availability of a directory of scripts I only need to use occasionally (I don't like to have them generally available, as they clutter up some common tab completions).
if [[ $PATH =~ $pathadd ]]; then
echo "myscripts off"
echo "myscripts on"