LEFT or SUBSTRING with a starting position of 1 are fine.

Investigate using the CHARINDEX or PATINDEX functions to find the point at which you want to cut off the string, e.g.

LEFT(ColumnName, PAT/CHARINDEX(Params))

SUBSTRING(ColumnName, 1, PAT/CHARINDEX(Params))

Without knowing the rules you're using to determine where to trim the string it's difficult to be more specific.

Cheers