You can use Replace
function as;
REPLACE ('Your String with cityname here', 'cityname', 'xyz')
--Results
'Your String with xyz here'
If you apply this to a table column where stringColumnName, cityName both are columns of YourTable
SELECT REPLACE(stringColumnName, cityName, '')
FROM YourTable
Or if you want to remove 'cityName'
string from out put of a column then
SELECT REPLACE(stringColumnName, 'cityName', '')
FROM yourTable
EDIT: Since you have given more details now, REPLACE
function is not the best method to sort your problem. Following is another way of doing it. Also @MartinSmith has given a good answer. Now you have the choice to select again.
SELECT RIGHT (O.Ort, LEN(O.Ort) - LEN(C.CityName)-1) As WithoutCityName
FROM tblOrtsteileGeo O
JOIN dbo.Cities C
ON C.foo = O.foo
WHERE O.GKZ = '06440004'