There are a lot of different options here :
Just by adding the coma in the search string to be replaced and use Trim
to get rid of spaces :
s1 = Trim(Replace(mystring, "site,", ""))
Specify the number of time you want the string to be replaced (first "1" is the start, the second for the number of replacements)
s1 = Trim(Replace(mystring, "site,", "",1,1))
Or the hard/bad way, to decompose your string in two pieces after the first occurence and then recombine to get result...
TempStart = Left(mystring, InStr(1, mystring, "site") + Len(mystring) + 1)
TempEnd = Replace(mystring, TempStart, "")
TempStart = Replace(TempStart, "site", "")
mystring = CStr(TempStart & TempEnd)