Use this function for the object numbers of all of your level objects.
function FreeObject()
repeat
inc i
if object exist(i)=0 then found=1
until found
maxnumobj=i
endfunction i
Then when you want to delete the level just use a function like this...
Function Delete_level()
for i=1 to maxnumobj
if object exist (i)=1 then delete object i
next i
endfunction
Then load everything for your next level using the free object function. It might not be the best way of doing it but it'll work.
Windsept
EDIT
When you are doing all of the deleting and loading you should load a fancy loading screen to make the transition between levels nice.