myList <- c("at","ax","CL","OZ","Gm","Kg","C100","-1.00")
myText <- "This is at Sample ax Text, which CL is OZ better and cleaned Gm, where C100 is not equal to -1.00. This is messy text Kg."
escape <- function(s) { return(gsub("([{[()|?$^*+.\\])", "\\\\\\1", s)) }
pat <- paste0("\\s*(?<!\\w)(?:", paste(sapply(myList, function(t) escape(t)), collapse = "|"), ")(?!\\w)")
cat(pat, collapse="\n")
gsub(pat, "", myText, perl=TRUE)