code:

-- work table
create table #process (
			id		int identity
			USERNAME	varchar(32)
			SYSTEMDATE	datetime
		       )

-- fill it with your data
insert into #process
select username, systemdatae
from ORIGINAL_TABLE

-- get your poo poo together
declare @x int
		, @y int
		, @currentuser varchar(32)
		, @currentdate datetime
		, @currentdate30 datetime

-- start and end points
select @x = (select min(id) from #process)
select @y = 1+(select max(id) from #process)


while @x < @y

BEGIN

select @currentuser = USERNAME
		,@currentdate = SYSTEMDATE
from #process
where id = @x

select @currentdate30 = dateadd(30, ss, @currentdate)

-- Now, remove the stuff you don't care about from your result set!

delete from #process
where USERNAME = @currentuser
and systemdate between @currentdate and @currentdate30

-- not just the next value, as you may have deleted it!
select @x = (select min(id) from #process where id > @x)

END