If your second string variable, STR2 holds consecutive strings, you can remove it by using STR3=System.Text.RegularExpressions.Regex.Replace(STR1,STR2,"")
or
STR3=STR1.Replace(STR2,"")
If you would like to remove discontinuous pattern, you can use a string array contains each string elements to be removed. Loop it through each elements in the array and use any of the above methods to remove the array elements.